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.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.