WASHINGTON -- With left-hander Odalis Perez ailing with shoulder tendinitis and the Nationals not wanting left-hander John Lannan to pitch on short rest, the team has decided that right-hander Garrett Mock will pitch on Sunday against the Giants and right-hander Tyler Clippard will pitch the next day, according to a baseball source.

The Nationals declined to make the announcement or say who will be taken off the roster to make room for the two pitchers. Both pitchers are considered highly-touted prospects and are playing for Triple-A Columbus.

Mock, who was acquired from the Diamondbacks in the Livan Hernandez trade in August 2006, is finally over his left knee injury. He is 3-2 with a 3.06 ERA in 11 starts. He has struck out 51 batters in 62 1/3 innings.

Clippard, who was acquired from the Yankees last December, has also been steady for Columbus, going 4-4 with a 3.41 ERA. He has struck out 59 batters in 60 2/3 innings.

Perez is expected to miss a turn on Sunday because of his tendinitis. Lannan was supposed to pitch on Tuesday. However, a rain postponement that day forced Lannan to pitch in the first game of a day/night doubleheader against the Cardinals on Wednesday.