Macha said "it is highly likely" that Bush will pitch in the subsequent series against the Cubs at Miller Park. Club officials will meet after the Cardinals series to make some decisions.
"We have to see how these guys perform," Macha said. "Right now, we have the second-highest ERA in the National League and we're looking for somebody. If you want to step up, step up and claim a spot."
Some might say that Bush did just that on Tuesday, when he fell into a 3-0 hole in the first inning but allowed no more runs through the sixth, when the blister cut his night short.
"We're not picking on Bush, OK?" Macha said. "We've not given Manny a fair shake. We asked him to come in in relief in the extra-inning game at Minnesota [on May 22] and that eliminated the start he was going to get the next day. So this is how this thing fit. We're not picking on Bush at all. Nobody has said he's out of the rotation or in the bullpen. He will be there for a couple days and we'll see how the things fit after Sunday."
The Brewers will start a left-hander in all three games against the Cardinals, who entered Thursday batting .243 with a .674 OPS against left-handed pitchers and .272 with a .782 OPS against righties.
Bush is 2-6 with a 6.45 ERA in nine career starts against St. Louis including 0-3 with a 10.57 ERA at Busch Stadium. Compare that to Parra, who is 2-1 with a 2.97 ERA in 12 games, including 10 starts, against the Cardinals and is 2-0 with a 2.86 ERA in five starts at Busch Stadium.
In his two starts this season, Parra has lasted only seven total innings.
"I'm excited any time I get to start," Parra said. "But other than the fact I'm going to start Sunday, I know absolutely nothing. It really doesn't matter, because there's no reason for me to think about anything else."
Bush was asked whether he was upset.
"I don't get upset a lot, so I would say no," he said. "But I don't have any idea what's going on. It was sort of vague as an explanation, so I'm hanging in limbo right now other than knowing I won't be pitching Sunday."