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9/1/2014 7:56 P.M. ET

Reliever Barrett among Nats' September callups

LOS ANGELES -- The Nationals made their September callups on Monday, recalling pitchers Aaron Barrett , Xavier Cedeno and Blake Treinen, catcher Sandy Leon and first baseman/outfielder Tyler Moore from Triple-A Syracuse. Outfielder Steven Souza Jr. (left shoulder) also returned to the Major League club after a rehab assignment.

Before being sent to Syracuse in early August, Barrett, 26, was an integral part of the Nationals' bullpen, going 3-0 with a 3.21 ERA in 40 games. His 11.50 strikeouts per nine innings ranks second among all rookie National League relievers this season, behind only Philadelphia's Ken Giles (12.72).

Barrett was sent down for a month to fix his throwing motion and to get some rest. On July 1 against the Rockies, Barrett was pitching to Justin Morneau and home-plate umpire Joe West called a balk, citing the right-hander's weird motion. The call allowed Troy Tulowitzki to advance to second base.

Despite an argument from manager Matt Williams, West upheld his call. Barrett was forced to change his motion in the middle of the inning, removing the hitch from his routine. Barrett went into a slump after that, allowing six runs in 5 2/3 innings.

Look for Barrett to continue his role in the late innings.

"He was in uncharted territory , so we wanted back him off," Williams said. "We allowed him to have more of a regular schedule [in Syracuse] and just keep him fresh. He is full-go right now. He will have the same role like he did before. He will become the wipeout righty. ... So he is fresh and ready to go."

Cedeno rejoins the Nationals for the fourth time this season, and he will be a situational lefty. He has not allowed a run in 3 1/3 innings this season.

Leon, in his fourth stint with the club this season, becomes Washington's third catcher behind Wilson Ramos and Jose Lobaton.

Moore will come off the bench in his third stint with the Nationals this season. During his Minor League season, Moore hit .265 with a .367 on-base percentage and a .434 slugging percentage at Triple-A.

Treinen rejoins the Nationals for the sixth time this season. He is 1-3 with a 2.35 ERA in 11 Major League games (five starts). As a starter in the Majors, Treinen has a 2.50 ERA, and as a reliever, he's posted a 1.54 mark. Treinen had an 8-2 record with a 3.35 ERA in 16 starts at Triple-A in 2014.

Souza returns to the active roster after missing 20 games with a left shoulder contusion. He was named the International League MVP last week, leading the league in almost every offensive category. 

Souza is 1-for-12 (.083) with one walk and four strikeouts in the Major Leagues this season. He will come off the bench as a pinch-hitter and defensive replacement.

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