WASHINGTON -- As expected, the Nationals recalled John Lannan, Sandy Leon and Eury Perez as Major League rosters expanded to 40 players on Saturday.

Mark DeRosa was also reinstated from the 15-day disabled list after he missed the last 25 games with an abdominal injury. The 37-year-old utility man is batting .152 (10-for-66) with six RBIs in 34 games.

Lannan has been an expected September callup for much of the season, especially with Stephen Strasburg's shutdown looming later this month. He made two starts with the Nats earlier this season -- both in doubleheaders -- and went 2-0 with a 3.46 ERA, seven strikeouts and seven walks in 13 innings.

Manager Davey Johnson said he doesn't have a specific plan in mind for Lannan yet, though he will eventually slide into the rotation. Should the Nats reach the postseason, the team could go with a four-man rotation. That, though, is a decision Johnson won't be making quite yet.

"I think he's going to throw a side [session on Sunday]," Johnson said. "I wouldn't pitch him for two more days anyway, because that would be his regular time to start. At this length of year, he had a good game last time out. I'm not even concerned about his workload for the next eight days."

At Triple-A Syracuse this season, Lannan went 9-11 with a 4.30 ERA. The 27-year-old left-hander also blossomed in his final two starts with the Chiefs, throwing two straight shutouts to end his season in the Minors. He was named the International League Pitcher of the Week last week.

Johnson said Lannan could soon pitch out of the bullpen before joining the rotation. On Thursday, Johnson said Strasburg has "a few more starts" this season, either two or three.

Perez, meanwhile, joins the Nats after spending the season with the Gulf Coast Nationals, Double-A Harrisburg and Triple-A Syracuse, batting .314 with 42 RBIs overall. Ranked No. 5 among the Nationals' top prospects, the 22-year-old outfielder will be used primarily as a pinch-runner. Johnson joked Perez will replace Edwin Jackson -- not as a pitcher, but as a late-game, last-ditch pinch-runner.

"I'm not big on trying to steal a base," Johnson said. "I don't care how good you are, you can still get thrown out. It's nice to have that option. With Edwin, I've used him -- I don't know how many times -- but there's been some other times I've used him and he didn't even have his shoes on.

"A few times, I can run Eury out and give him an at-bat, lead off an inning or pinch-hit, play center field. Stuff like that."

Leon rejoins the Nats after batting .250 in 10 games with the team earlier in the season, mainly in July. Leon batted .322 with Class A Auburn, Harrisburg and Syracuse this season.

The 23-year-old will be used as the third catcher behind Kurt Suzuki and Jesus Flores, as well as a pinch-hitter.

Overall, Johnson said the Nationals didn't elect to call up many prospects because of the team's success this season. The Nats were the second team to reach 80 wins this season and entered Saturday with a 6 1/2-game lead over the Braves in the National League East.

"I've been in this situation before, it's not my first rodeo," Johnson said. "We have what we need here without any reinforcements. These other guys get some big league time, some big league experience.

"We're no longer developing, in case you haven't noticed."

Werth expects to be OK after exiting with cramps

WASHINGTON -- Nationals right fielder Jayson Werth exited Saturday's 10-9 loss to the Cardinals prior to the ninth inning with a hamstring cramp.

With the score tied at 9 as the Nationals took the field, Eury Perez -- called up earlier in the day as rosters expanded to 40 players -- made his Major League debut in center field and Bryce Harper moved to right as Werth remained in the dugout.

Werth finished the game 1-for-5 with one run scored and two strikeouts, lowering his batting average to .313.

Perez never got to bat, though he did field a fly ball for the final out of the ninth.

"It just felt intelligent not to play any longer, so [manager Davey Johnson] took me out," Werth said. "It's been a long time since I've cramped up during a game, so hopefully it'll be nothing and I'll be back by [Sunday], but we'll see. I don't think it's too serious, though."

Werth added it's been "four or five years" since he's cramped up during a game, but Johnson confirmed the issue wasn't anything serious.

"He was cramping real bad," he said. "He didn't pull anything, so he should be OK."

Johnson 'not worried' about Zimmerman's throwing

WASHINGTON -- Between the manner in which Ryan Zimmerman has rebounded from his shoulder ailments earlier this season and his continued stellar defense at third base, there isn't much the Nationals can nitpick with their third baseman.

After seeing his batting average drop as low as .218 in late June as he battled a sore right shoulder, Zimmerman has ridden a second-half tear to boost that clip to .282 entering Saturday with 17 home runs and 69 RBIs.

But after seeing Zimmerman revert several times to a sidearm motion while throwing to first base, manager Davey Johnson was asked about his preference for the third baseman. Zimmerman has 11 errors on the season, fourth most among NL third basemen -- though he could have several more without the stellar fielding of Adam LaRoche at first base.

"I like it when he cuts it loose more," Johnson said of Zimmerman's throwing motion. "Yeah, that's the shoulder he's had the problem with the [cortisone] shots, so I'm sure there's probably a little impingement in there or whatever. When there is, and you don't get [your arm] up there and cut it loose, then you're going to have some arm problems as well.

"But he's awfully good the way he is, I'm not worried about it."

Johnson said he hasn't discussed the issue "one iota" with Zimmerman, though he did reach back into his memories of coaching the Mets in the 1980s to explain the significance of the issue.

"I had a guy that had a little problem throwing to first, it was a little switch-hitter I had in New York, Howard Johnson," Johnson said. "I said, 'Look, I want you to just throw it over there. As hard as you can, and I don't care if it lands 15 rows up, just fire it. Don't be aiming nothing over there, just cut it loose.' I don't worry about things like that, just as long as they get it and throw it."

Worth noting

• The Nationals had the National League's best record in August at 19-10. That .655 winning percentage is the Nationals' best single-month mark of the season.

• The Nats entered Saturday tied with the Rangers for the Major League lead in run differential at +113.

• Bryce Harper carried a four-game hitting streak into Saturday's game with the Cardinals and is 11-for-33 (.333) over his last eight games.