After being named to the National League All-Star squad earlier in the day, Gallardo left Sunday's game in the bottom of the third inning with a strained left oblique muscle.
Gallardo was still holding out hope of a quick return before a Monday visit with the team doctor, but Macha inserted a dose of reality. Macha said Gallardo would probably not make another first-half start and would definitely not pitch in next week's All-Star Game.
"Throwing in the All-Star Game? No," Macha said. "I'm saying that, best-case scenario, he's probably going to be doing no baseball activities for at least 10 days. ... He's not going to pitch in the All-Star Game. You can't have a guy walk off the mound one week and then put him in [All-Star] Game the next. No."
The Brewers summoned outfielder Lorenzo Cain from Triple-A Nashville, and he was making his way to Milwaukee on Monday morning. According to Brewers general manager Doug Melvin, the idea behind bringing Cain to Milwaukee sooner rather than later was to avoid any potential logistical issues if Gallardo were to go on the 15-day disabled list.
Gallardo was evaluated by team physician William Raasch on Monday afternoon, and the strained left oblique muscle was confirmed. Officially, the team has listed Gallardo as day-to-day and the Brewers determined they would not make a roster move Monday.
If the doctors had determined that Gallardo would have to go to the DL, then the team would have activated Cain.
In that scenario, Gallardo would be eligible to return from the DL beginning July 20 and would miss a minimum of two starts.
"That's the tough thing, whether to put him on the DL or not," Melvin said. "If you don't put him on the DL and you use the days, he could be ready the day after the All-Star break. But if you don't pitch him this week, you've got seven days and you've got [three] days at the break, that's  days.
Gallardo said he was feeling better Monday morning, though he described the oblique as still feeling tight. He described it Monday as feeling initially like a "cramping" sensation just before he exited the game Sunday.
Even if he's unable to pitch, Gallardo said he planned to make the trip to Anaheim for All-Star festivities.
"He's feeling decent, but this is something that's [to be taken seriously]," Macha said. "I had Tim Hudson and he would get them every once in a while, and it's a long process coming back if there is a tear in there.
"So [the doctors] have to figure out what's going on in there [with Gallardo]."
Though he is sidelined by injury and may be forced to watch from the dugout, Gallardo remained enthusiastic about his first All-Star Game.
"I'm still excited about it," Gallardo said. "It's one of those things that we all look forward to. Hopefully I'm available, but we'll see."
Jordan Schelling is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.