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4/13/2014 12:21 A.M. ET

Werth misses start with groin cramp, pinch-hits

ATLANTA -- Outfielder Jayson Werth left Friday's 7-6, 10-inning loss to the Braves in the bottom of the 10th because of a minor cramp in his groin. He was not in Saturday's starting lineup, but he came in as a pinch-hitter in the eighth inning and hit a hard single before the Nats fell, 6-3.

"He cramped last night a little bit on a swing [against] Craig Kimbrel and let us know at the end of the inning when he was running back to the field that he felt it," manager Matt Williams said. "So, again on the side of caution, we said 'We'll get you out of there. We'll re-evaluate tomorrow.' I think he's fine, but at this point, looking at the long term, we don't want to push him either. So that's why he's not in there."

Werth is one of the best hitters on the Nationals this season, going 14-for-38 (.368) with two home runs and eight RBIs.

Zimmerman out 4-6 weeks with broken thumb

ATLANTA -- Nationals third baseman Ryan Zimmerman will miss four to six weeks after fracturing his right thumb during a 6-3 loss to the Braves on Saturday night at Turner Field. It will mark the fourth consecutive year Zimmerman will be placed on the disabled list, which is expected to happen on Sunday.

With the Braves leading, 4-1, in the fifth inning, Zimmerman hit a one-out double before being picked off by left-hander Alex Wood. Zimmerman argued with second-base umpire Paul Nauert over the call, but to no avail. The Nationals had already lost a challenge.

Zimmerman then looked at his right hand and was slow to get up. He then slowly walked into the dugout and was taken out of the game.

With Zimmerman out, it means that starting second baseman Anthony Rendon will now play every day at third, while Danny Espinosa will be the starting second baseman.

"It's not our first choice, certainly. The fact that they can play multiple positions is good in times like this," manager Matt Williams said. "You certainly don't want to miss someone like Zim for that amount of time, but it is what it is. There is nothing we can do about it now except play."

According to a baseball source, the Nationals are expected to recall infielder Zach Walters from Triple-A Syracuse on Sunday. Ranked by MLB.com as the Nationals' No. 10 prospect, Walters will be a backup and can play multiple positions, including shortstop and second base. While no one has questioned his offense, the Nationals want him to get better defensively. It was one of the reasons he was sent down in Spring Training.

It's only the second week of the regular season, and the Nationals have been hit hard with injuries. Catcher Wilson Ramos is out four to eight weeks because of broken left hamate bone. Right-hander Doug Fister has yet to play a regular-season game because of a lat strain, and Denard Span was placed on the seven-day DL on Saturday because of a concussion.

"It is what it is. Nobody is going to feel sorry for us," Williams said. "[Losing Zimmerman] is a big one, but we don't have a choice. We have to go play, compete and win games. We have to do that starting tomorrow."

Span to 7-day DL; Souza, Treinen come up

ATLANTA -- The Nationals were busy before Saturday's game against the Braves, placing Denard Span on the seven-day disabled list with a concussion, while recalling outfielder Steven Souza Jr. and selecting the contract of right-hander Blake Treinen from Triple-A Syracuse. Right-hander Aaron Barrett was optioned to Syracuse.

The injury to Span occurred in the top of the eighth inning of Friday night's 7-6 loss to the Braves in 10 innings. With runners on first and second and two outs, Span singled to center field off right-hander David Carpenter, scoring Bryce Harper. Span was awarded second base for colliding with second baseman Dan Uggla, who was charged with interference.

Span was down for the count for several seconds before being attended by head athletic trainer Lee Kuntz and manager Matt Williams. Span remained in the game, but acknowledged afterwards that he had a mild headache. He took a battery of tests, and it was revealed on Saturday that he suffered a mild concussion.

The Nationals erred on the side of caution by putting Span on the DL. He has a history of concussion problems. In fact, Span missed a good portion of the 2011 season after he was involved in a home-plate collision with then-Royals catcher Brayan Pena.

"Given his history, we thought we would go ahead and get him on the seven-day DL," Williams said. "We just want to make sure."

With Span on the DL, Nate McLouth will get most of the playing time, while Souza will come off the bench. Souza, ranked by MLB.com as the 14th-best prospect in the Nationals' Minor League system, was hitting .273 with a .429 on-base percentage and a .545 slugging percentage in seven games for Syracuse this season.

"He's been playing well. It's a question of what our needs are, certainly on the team," Williams said. "It's one of the reasons that Nate's here, because when things like this happen, he can step right in and fill in for anybody at any position in the outfield. But Steven has been playing well, and he played well this spring. There's nothing other than that, other than we thought he was a good choice."

As for Treinen, he made one start for Syracuse, but he will be used as a reliever because the bullpen is taxed. He didn't have to wait long to make his Major League debut, entering in relief of Taylor Jordan in the sixth inning during Saturday's 6-3 loss to the Braves and tossing two shutout innings, allowing two hits and striking out one.

Treinen said he wanted to leave a good impression before Spring Training ended, and it's safe to say he did. After getting hit hard in his first relief appearance, Treinen allowed only one run in his last 8 2/3 innings and notched seven strikeouts.

Treinen -- who ranked as the 17th-best prospect in the Nationals' Minor League system -- can give the Nationals more than one inning.

"I got a phone call between 11:30 and 12 last night from manager Billy Gardner telling me to be ready to leave in the morning for Atlanta," Treinen said. "So I just called my family, called my fiancée and called my agent and kind of worked out the details with them, and I think they're on their way down now.

"Internally, I was screaming. Externally, I was pretty relaxed. I think it took me until I went to bed when I realized, because then it's hard to sleep, but I feel like I'm ready. I'm excited. Performance-wise, I feel like I'm here for a reason."

Treinen's arrival meant Barrett had to be sent down. It sounded like it was hard for Williams to demote Barrett, who is a closer by trade and is not known to pitch multiple innings. Besides, Barrett is dealing with a minor finger injury, so he is expected to get some rest before he gets back on the mound for Syracuse.

"The issue with Aaron is he has done everything we've asked him to do -- above and beyond," Williams said. "But the way our bullpen is situated right now, we needed a guy that could give us length. It's one of those decisions that you make because you have to make it -- not because you want to make it. So Aaron was understanding. This is the way the game is sometimes."

Barrett understood the bullpen's plight.

"If you look at it logistically, the bullpen has been pretty overworked at the moment. They just told me right now they need innings. It is what it is. Yeah, it stinks getting sent down, but at the same time, I'm not going to beat myself up about it. It's not because it's performance-based. We need innings."

Additionally, infielder/outfielder Jeff Kobernus was recalled from Syracuse and placed on the 60-day disabled list with a left hand fracture.

Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the Time. He also can be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.