VIERA, Fla. -- The Nationals made their third round of roster cuts Monday, as they optioned right-hander Collin Balester and left-hander Atahualpa Severino to Triple-A Syracuse; right-hander Ryan Mattheus and lefty Aaron Thompson to Double-A Harrisburg; and right-hander Juan Jamie to Class A Potomac. Washington also released left-hander Ron Villone.

With the moves, the Nationals' Major League roster is now down to 45 players.

Of the names moved off the roster, Balester, Thompson and Villone were the most intriguing. Villone was a favorite of manager Jim Riggleman. Last season, Villone was the veteran leader of the bullpen, pitching in 63 games and compiling a 4.25 ERA. This spring, the left-hander was hit hard, allowing three runs in 1 2/3 innings.

"That was tough, not just for me but for [general manager Mike] Rizzo, [pitching coach] Steve McCatty and a lot of people in the clubhouse in general," Riggleman said. "He is such a class act. He is a wonderful guy. He was an effective pitcher for us last year."

With Villone gone, the Nationals most likely will carry only one lefty reliever -- Sean Burnett -- on the 25-man roster, even though left-handers Jesse English and Doug Slaten are still in camp.

Riggleman has often said he is confident that right-hander Tyler Clippard can get left-handed hitters out. Lefties hit .122 against Clippard last year.

Thompson, who was acquired from the Marlins in the Nick Johnson trade last July, pitched four shutout innings and impressed the big league staff, but he wasn't considered part of the mix. Riggleman hinted that right-hander Garrett Mock has been impressive and most likely will be in the rotation. Mock hadn't walked a batter in five innings entering Monday's action.

"It's very important to him to pitch to contact, and that is not easy for him," Riggleman said of Mock. "When your ball moves a lot, sometimes it could move out of the strike zone. But he has been able to keep his movement in the strike zone. Even when he doesn't throw a strike, it has the appearance of a strike. So hitters might swing at it. That's huge."

Livan Hernandez is considered the leading candidate to take the fifth spot in the rotation. John Lannan, Jason Marquis and Scott Olsen have already secured spots.

Inconsistency is the reason Balester was sent down, after he gave up four runs in four innings. Rizzo still believes Balester has the pitches to get people out.

"The stuff is there," Rizzo said. "He has a good fastball and a good breaking pitch at times. We still have high hopes for him. He is going to go down and master his craft, get him in the strike zone a little better and be more consistent."