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6/21/2013 11:25 P.M. ET

Harper set to begin rehab assignment Tuesday

WASHINGTON -- Nationals manager Davey Johnson said Friday that outfielder Bryce Harper is on track to make a rehab appearance at Class A Potomac on Tuesday.

Harper has been on the disabled list since June 1 with bursitis in his left knee, but he has not had any additional swelling in the knee since resuming baseball activities in the past couple of days, according to Johnson. Before Friday night's game, the 20-year-old took some swings in the batting cage, both off a tee and against live pitches. He also did some light work in the outfield.

"[Harper] is doing fine," Johnson said. "I was in the workout room with him, I saw him doing contortions and everything else. … He'll probably work with us through Monday, [doing] baseball stuff, before he'll be ready to go out."

Harper has said that he wanted to make a rehab appearance before returning to the Nationals' lineup. When asked about how much time Harper would need to spend in the Minor Leagues, Johnson said, "Knowing Harp, I don't think he's going to need much."

Potomac's current homestand ends on Thursday.

When Harper returns, he will wear a protective knee pad to prevent any further damage to the knee.

"It's not so much timing as it is just seeing how he feels after [games], how the knee reacts to being out there for nine innings," Johnson said.

Zim's glovework aids Strasburg's victory

WASHINGTON -- Ryan Zimmerman's defense has faltered at some inopportune moments this season, but the Nationals' third baseman helped preserve Friday's 2-1 win over the Rockies with some fancy glovework.

Zimmerman's sparkling defensive play in the seventh inning came at a critical juncture in the game. Zimmerman's RBI double tied the game, 1-1, in the previous frame and the Rockies' Josh Rutledge stood at second with two outs, representing the go-ahead run.

Pinch-hitter Tyler Colvin lofted a 2-0 pitch from Stephen Strasburg down the left-field line, and Zimmerman -- shaded toward second base against a left-handed hitter -- turned and gave chase as the ball twisted into foul territory. Zimmerman looked over his left shoulder to spot the ball, then followed its path until he was looking straight backward as he ran.

"I saw it," he said. "I was running, and then at the end you just kind of turn and you wait until the last second, because obviously you just kind of put your glove where you hope it's going to be."

Zimmerman stuck his mitt straight out in front of him and snagged the pop fly to end the inning.

Asked how often he works on that sort of play, Zimmerman first joked, "Every day," before revealing the truth.

"No, you just kind of go after it and run where you think it's going to be, and hopefully it falls into your glove," he said. "You've got to be lucky sometimes.

"It's a fun catch to try and make. If you don't make it, it doesn't matter. If you make it, it's great. It does matter, but nobody expects you to make it."

Zimmerman's defense, particularly his throwing, has been a hot topic at times this season. The 2009 National League Gold Glove Award winner has made 11 errors in 57 games, and twice this season unearned runs from those miscues have factored into losses for Strasburg.

This time, his play saved Strasburg from having to make any more pitches with a runner in scoring position. The right-hander, who picked up his fourth victory, gave an emphatic fist pump and shouted as he watched the grab.

"That was awesome," Strasburg said. "Just watching the ball kind of hang up in the air, and he had to run a long ways and kind of snare it at the last second. So, that was huge."

Zimmerman also made a nice play in the fourth inning. With two outs and nobody on base, Todd Helton slapped a hard grounder down the third-base line. Zimmerman went to his right, smoothly vacuumed up the ball as he slid on one leg, then got up and slung a throw across the diamond. First baseman Adam LaRoche helped him out with a nice scoop to easily beat Helton.

Span's swing shows signs of progress in win

WASHINGTON -- In the Nationals' 5-1 win over the Rockies on Thursday, Denard Span put together the type of game Washington was looking for when it acquired him from the Twins this winter to take over the leadoff spot. After a strikeout in his first at-bat, Span reached safely in each of his next three trips -- with a single, double and walk -- and also stole a base.

But that type of performance has been in short supply, at least since the first two weeks of the season. While Span has provided the stellar center-field defense the Nationals craved, his offense has lagged behind. The left-handed batter hit .313 with a .421 on-base percentage over his first 13 games with Washington, and .243 with a .285 OBP in 54 games since. His on-base percentage for the season dropped as low as .306 before Thursday's surge.

While Span has missed a few games with a sore foot, Nationals manager Davey Johnson has continued to bat him leadoff on a daily basis when he's been available. Johnson has considered moving Span down in the order, but the Nationals rank 29th in the Majors in on-base percentage, with many players struggling to reach base.

"I think about all kinds of options," Johnson said. "But not many are getting on as much as he is, even when he's in a slump."

Span's sore right foot is part of the problem. For one thing, he said Friday he continues to deal with some soreness and inflammation that requires icing it daily.

The bigger issue is the cause of the injury -- three different fouls balls he has hit off the same spot. For Span, that's an indication that his swing is not right. He can see on video that his hands are getting around the ball instead of staying inside it, and he can feel on particular swings that something is off.

"If I swing at a certain pitch and foul it off a certain way, or if I miss and pop it up a certain way, I can definitely tell what I'm doing," Span said. "And that's the tricky thing about it. Even though I can feel what I'm doing, it's not that easy to fix. … The muscle memory, the amount of times I've practiced that bad habit, it's kind of been ingrained."

The issue might not be only with how Span swings, but also when. According to PITCH f/x data, Span is seeing a career-low percentage of pitches within the strike zone this season, while offering at a career-high percentage of pitches outside the zone.

"Part of that is my swing not feeling great," he said. "The ball is sped up, everything else is sped up, and at times I swing at balls that aren't in the zone, and I think that is a direct link. I'm not 100 percent sure, but I think that is somewhat of a link. Usually when a hitter is on time and under control, they swing at better pitches and lay off tough pitches."

Even though this is Span's first season in Washington after five with Minnesota, he has continued a career-long trend of struggling on the road. The 29-year-old hits 60 points higher at home overall and a whopping 128 points this season (.325 at Nationals Park, .197 away).

Fortunately for Span, the Nationals just began a stretch in which they play 33 of 48 games at home. On Thursday, he reached safely in three plate appearances for only the eighth time and made some of his most solid contact all year. In the fifth inning, he squared up a high fastball from Roy Oswalt and drove it off the wall in right-center field, coming within a few feet of his first home run.

"It definitely was one of the best balls I've hit in a while, so it felt good," Span said.

Span, who brought a solid .357 OBP into this season, is confident he will find a way to produce. In the meantime, he appreciates Johnson sticking with him, something that doesn't sound likely to change anytime soon.

"I'm not worried about him," Johnson said. "He's a gamer. He hasn't been pleased with how he's been hitting, but he still manages to get on base and he covers the outfield like nobody else."

Mattheus about four weeks away from return

WASHINGTON -- Although the broken bone in his right hand has healed, reliever Ryan Mattheus said that he is still about four weeks away from returning to the Nationals' bullpen.

Mattheus played catch again on Friday after throwing for the first time since the injury on Thursday afternoon. While he has yet to throw at his regular velocity, Mattheus said that the hand feels fine and hasn't given him any issues so far.

"I've been pretty surprised with how it's reacted," he said. "It hasn't gotten any added stiffness, any added soreness or anything like that with the two light days of throwing. So, I've been pretty happy with it."

Mattheus broke the hand by punching his locker after a rough outing on May 19. In the month since, he has worked with the Nationals' training staff to maintain his arm strength through manual-resistance exercises. He also recently practiced squeezing his hand in a bucket of rice to improve his grip.

Mattheus said that he will continue to throw harder and from farther distances before returning to a mound, first in bullpen sessions and then in a Minor League rehab assignment. If all goes well, he thinks the process will take four weeks.

"I don't know exactly, they haven't told me exactly," Mattheus said. "I hope it'll be a little sooner, that's just my own timetable, but obviously I want to push it more than they really can. That's something the trainers and the doctors are going to have the final say in."

Mattheus called the injury "one of the most frustrating things I've dealt with" as a baseball player. He is eager to put the incident behind him.

"I've been hurt before, but that kind of stuff was uncontrollable," Mattheus said. "This was something that was self-inflicted, a stupid decision. So it's been tough to swallow."

Worth noting

• The Nationals on Friday agreed to terms with three more selections from the First-Year Player Draft, including fourth-rounder Nicholas Pivetta out of New Mexico Junior College. Fellow right-handers John Simms, an 11th-rounder from Rice, and Andrew Cooper, a 12th-rounder from Sierra (Calif.) Junior College, also signed.

Washington now has struck deals with 13 of its top 14 picks, missing only 13th rounder John Costa, a righty from Palm Beach (Fla.) Community College.

Pivetta, one of three righties the Nationals took among their top four picks, is a 6-foot-5, 215-pounder whose fastball has reached the upper 90s when appearing as a reliever.

• Manager Davey Johnson said that Roger Bernadina (eye) and Chad Tracy (back spasms) are both healthy after receiving treatments on Thursday.

Tom Schad is an associate reporter for MLB.com Follow him on Twitter @Tom_Schad. Andrew Simon is a reporter for MLB.com. Follow him on Twitter @HitTheCutoff. This story was not subject to the approval of Major League Baseball or its clubs.