Nats GM Mike Rizzo isn't worried about the team's injuries.

ATLANTA -- It's the third week of the regular season and the Nationals have lost three players -- Denard Span, Wilson Ramos and Ryan Zimmerman -- from the starting lineup due to injuries.

However, general manager Mike Rizzo doesn't seem to be worried. He believes the team has enough depth to make up for the loss of those three players.

Anthony Rendon replaces Zimmerman at third base, and Nate McLouth is expected to get most of the playing time while Span is out with a concussion. Early in Spring Training, the Nationals acquired Jose Lobaton from the Rays, and he is now playing every day behind the plate.

"We feel as good as we can be with the glut of injuries that we had to key players, and you are talking about your middle-of-the-lineup bats that are out," Rizzo said. "We feel as good as we can. ... When you lose two middle-of-the-lineup bats, we feel good about the roster."

Walters called up as Zimmerman hits disabled list

WSH@ATL: Zimmerman leaves game with fractured thumb

ATLANTA -- The Nationals placed third baseman Ryan Zimmerman on the 15-day disabled list because of a fractured right thumb and recalled infielder Zach Walters from Triple-A Syracuse on Sunday.

Zimmerman was off to a great start, batting .364 with two home runs, six RBIs and a .405 on-base percentage before he fractured the thumb in the fifth inning of Saturday's 6-3 loss to the Braves. After hitting a double to put runners on second and third with one out, Zimmerman was picked off by left-hander Alex Wood. Zimmerman argued with second-base umpire Paul Nauert over the call to no avail.

Zimmerman then looked at his right thumb and was slow to get up.

"When I looked down, it didn't look normal," Zimmerman said. "I [have] never really broken a bone or done anything like that, but you could tell something wasn't normal."

Zimmerman then slowly walked into the dugout and was taken out of the game. It marked the fourth consecutive year in which he was placed on the disabled list.

"It's tough, but that's part of sports, it's part of the game. No reason to sit there, pout or feel sorry for myself. Nobody else is," Zimmerman said. "It's a freak accident. It stinks. I was off to a good start and the team was playing well. It's just frustrating."

Zimmerman will go to the Cleveland Clinic on Monday to see a hand specialist and make sure that there was a clean break inside the thumb.

Walters, ranked by MLB.com as the Nationals' No. 10 prospect, will be a backup infielder. He can play all of the infield positions, except for first base. Before the promotion, Walters was batting .290 in nine games for Syracuse.

"[Walters] is capable of playing all three [infield positions]. His natural position is shortstop and he has the skill set to play shortstop," general manager Mike Rizzo said. "Being a switch-hitter off the bench with power helps us."

Careful Span taking time after sustaining concussion

WSH@ATL: Span puts Nats ahead with hard single

ATLANTA -- Nationals center fielder Denard Span said he feels much better, but he agrees with the decision to put him on the seven-day disabled list because of a concussion.

The injury occurred in the eighth inning of Friday night's 7-6 loss to the Braves. 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 also sustained a thigh injury on the play. He said the thigh injury hurts more than the concussion.

"I was trying to get to second base. I saw Uggla. I think he picked me up late," Span said. "Even with us colliding like that, I didn't think it would be an impact like that. I thought maybe we would have been able to brace each other.

"It's crazy, this game. You never know what's going to happen. ... I felt I got hit by a little stocky running back -- Maurice Jones-Drew."

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

Span rested most of Saturday at his hotel room and rejoined the team on Sunday. He is expected to take more physical tests Monday to make sure that he is OK.

Span learned his lesson from three years ago. He missed a good portion of the 2011 season after he was involved in a home-plate collision with then-Royals catcher Brayan Pena. Span admitted that he rushed back to play and paid the price. This time, he is taking his time before he gets back on the field.

"I feel fine. If I had to play, I could have played [on Sunday]," Span said. "But I understand with my history that it was best for me to go ahead and go on the seven-day DL just to be safe. I'm thinking about the rest of the season and the rest of my life."

Worth noting

• Right-hander Doug Fister will have another bullpen session on Monday in Miami, according to Rizzo. Fister is expected to throw all of his pitches during the session.

• McLouth was originally in the starting lineup on Sunday against the Braves, but he was a late scratch because of patellar tendinitis in his left knee. He was throwing in the outfield before the game when he felt pain.

McLouth has had the problem for years, but he has managed to play with it. He is expected to be out for at least a couple of days.