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8/20/2014 11:40 P.M. ET

Harrisburg's Rivero strong in second start off DL

Making his second start for Double-A Harrisburg since returning from a three-month-long stay on the disabled list due to an arm injury, left-hander Felipe Rivero, the Nationals' No. 17 prospect, threw five scoreless innings Wednesday. It wasn't enough for a victory, however, as Portland came back to defeat Harrisburg, 5-3.

Rivero struck out six batters, walked two and held the Sea Dogs to one hit. He threw 70 pitches and matched his longest start of the season.

Before his injury, Rivero opened the season with Harrisburg. He has made eight starts for the Senators, going 1-6 with a 5.18 ERA. He has struck out 25 batters and walked 16 in 33 innings.

Rivero was part of the package the Nationals acquired from the Rays in February in exchange for right-hander Nate Karns. Also included in the deal was outfielder Drew Vettleson, who went 0-for-4 with four strikeouts Wednesday for Harrisburg. The Nationals' No. 15 prospect also was limited by injury this season (a broken hand) and is hitting .227/.260/.409 in 64 games with the Senators.

-- Teddy Cahill

Williams eyeing September return for Zimmerman

WASHINGTON -- Nationals manager Matt Williams said Wednesday that his gut feeling for the return of injured third baseman Ryan Zimmerman is "sometime in September." It's been nearly a month since Zimmerman suffered a Grade 3 hamstring strain running to first base in Colorado.

"Sometime in September. When? I don't know, but sometime in September," Williams said. "I don't think anything would happen until after [Sept. 1], because he still hasn't gone [through] the full gamut of tests. And then he would have to have some time to play."

Where Zimmerman goes on his rehab assignment when the hamstring heals is still in question, though. If he remains sidelined until the middle of September, the Minor League season could be over, as the regular-season slates for the Nationals' Class A, Double-A and Triple-A affiliates wrap up on Sept. 1.

In that case, Williams said Zimmerman could potentially play in the instructional league in Florida, which starts Sept. 13. If it's before then, Williams said the team would stage simulated games for Zimmerman.

"Whether it's [at-bats], fielding balls or being in the outfield or being at third or any of that," Williams said, "we'll put him through the whole schmear and make sure he's ready to go."

Williams trying to gets Nats' regulars some rest

WASHINGTON -- Infielder Anthony Rendon was out of the Nationals' starting lineup Wednesday against the D-backs, sitting for the first time since June 9, as utility man Kevin Frandsen got the nod at third base. Rendon had made 62 straight starts for the Nationals before Wednesday, and manager Matt Williams said he made the decision to provide the second-year man with a day of rest.

"He needs a day," Williams said of Rendon, who entered Wednesday hitting .276 on the season with 16 homers and 67 RBIs. "He's played a lot of games. He's been on base a lot, of course, because he's hit well. Played hard defensively, too. So we'll give him today."

That was the plan, anyway. Rendon appeared as a pinch-hitter in the ninth, roping an RBI single to left field to give the Nats a 3-2 walk-off win over the D-backs, Washington's ninth straight victory.

Rendon is on pace to play in a professional career-high 157 games this season after appearing in 134 games (98 big league games and 36 Minor League games) in 2013, his second year in the Nats' organization.

He suffered ankle injuries while playing collegiately at Rice, and also professionally with Class A Potomac in 2012. His Draft stock in 2011 was hurt significantly because of lingering problems in his shoulder.

Now, it seems that Rendon has put those issues behind him.

"You're in unchartered waters," Williams said of the amount Rendon has played this season. "You don't have experience in that regard, when you get to that point. But I think he's held up really well. His legs have been great. He's stealing bases and hitting doubles and shown good power, all of those things that we talk about every day. But physically, I think he's held up pretty well. We have to be mindful of it, though. Make sure that we give him a day when we can."

Rendon's scheduled day off was part of an ongoing push from Williams to provide his starters with some much-needed rest, especially Ian Desmond, who's sat just two games all season and often refuses to take days off, according to Williams.

"At some point, I'm going to have to force him," Williams said.

Williams said he also plans on finding days off for left fielder Bryce Harper, center fielder Denard Span and first baseman Adam LaRoche over the next few weeks, as the Nationals are scheduled for games on every day except three until the end of the regular season on Sept. 28.

"That's the mindset right now, to try to intersperse those guys on occasion, where we can just give them a day to rest and get some work done with the staff and get recharged a little bit," Williams said. "The off-days are pretty sparse from here on out, so we've just got to make sure that we try to get guys days when we can."

Nats' Souza set to begin rehab assignment

WASHINGTON -- Outfielder Steven Souza Jr. (left AC joint sprain) confirmed he is starting a rehab assignment on Thursday. He'll begin the stint with Class A Hagerstown for one game before heading to Class A Advanced Potomac for three to four games.  
The prospect has been on the 15-day disabled list since Aug. 10 after crashing into the right-field wall at Turner Field two days earlier. He's eligible to return to the Nationals' 25-man roster on Monday. 
"It's a matter of just how I feel," Souza said. "Kind of build of slowly and see how I feel from there."
Souza expected to take live batting practice on the field at Nationals Park on Wednesday afternoon for the first time since suffering the injury, but rain forced the tarp on the field and ended the session early. Still, he went through a full hitting workout in the cage and reported feeling great afterward. 
Moving forward, Souza said there's no frustration stemming from getting hurt just four days after the Nationals called him up from Triple-A Syracuse. As unfortunate as it the situation is, the outfielder said he must keep his sights trained on the future.
"I don't think you have control of it. It definitely [stinks]. It's just not the way you draw things up. But life just doesn't draw out the way you want it to sometimes," Souza said. "You get knocked down, you get back up and keep going. That's just the way I've done things, it's the way I try to do things, and I don't look too much into the fact of I got here and I had a setback. … I've got an obstacle I've got to climb, and I'm going to keep going."

Worth noting

• Outfielder Nate Schierholtz, signed to Minor League contract on Monday, went 3-for-5 with a home run and four RBIs in his debut for Syracuse on Wednesday night.

Daniel Popper is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.