McDonald, 28, continued to say that he felt fine, according to Rob Biertempfiel of the Pittsburgh Tribune-Review. However, his shoulder got to the point where he could not pitch. He was originally slated to pitch in Tuesday's game, but the Pirates had to scratch him. In his place, Jeanmar Gomez will be bumped up to make the start.
Pittsburgh may not miss much in the short term with McDonald out of action. He has struggled this season and has a 5.76 ERA and league-leading 20 walks over his first six starts. His latest start on April 30 was his worst yet. Against the Milwaukee Brewers he allowed seven earned runs over five innings.
Harrison, 25, may only be on the roster temporarily as the Pirates will need to find a starting pitcher for May 11. Harrison has seen some playing time the last two seasons but has a career OPS of just .631. In the minors, he has consistently hit for a high batting average and gotten on base at a good rate.
Francisco Liriano is expected to be back with the Pirates soon and seems likely to take Saturday's now-open start.