The team on Thursday afternoon the promotion of Gallardo, its top pitching prospect, and left-hander Chris Capuano was placed on the 15-day disabled list with a strained left groin. Ideally, Gallardo will join the starting rotation Monday at Miller Park, but he may be needed sooner than that in the Brewers' worn-out bullpen.
The Brewers open a three-game series in Minnesota on Friday night.
"I don't know now," said manager Ned Yost, after he used four relievers for 3 1/3 innings of Thursday's 6-5 win over Detroit. "There's nothing I can do [on Friday] because Gallardo won't be available until Saturday, anyway. ... If I need him in a long situation, I will pitch him on Saturday or Sunday."
Gallardo, 21 years old and a right-hander, is 8-3 with a 2.90 ERA in 13 starts for Triple-A Nashville and leads all of Minor League Baseball with 110 strikeouts. If the Brewers can avoid using him out of the bullpen this weekend, Gallardo would make his Major League debut on Monday against the San Francisco Giants.
Capuano was scratched minutes before his scheduled start Wednesday night when he felt tightness in his groin. Carlos Villanueva made the emergency start and provided five bullpen-saving innings, setting up the Brewers for a comeback win over the Tigers.
Because Villanueva threw 77 pitches -- 33 more than his previous season high -- he will not be available until the Brewers return home next Monday to face the Giants. Gallardo last started for Nashville on Tuesday, so he will be available beginning Saturday, Yost said. If Gallardo pitches in relief, Villanueva would make the Monday start at Miller Park.
"I told Villanueva not even to put his spikes on until we get back home," Yost said. "He won't be available."
The 23-year-old Villanueva has flourished since earning a spot in the bullpen out of Spring Training. Including Wednesday's no-decision, he is 5-0 with a 2.87 ERA in 27 games.
Gallardo, the Brewers' second-round Draft pick in 2004, has lost his last two starts but pitched well in the most recent outing, allowing two earned runs in six innings at Memphis while striking out 12 batters.
In 73 career Minor League games, Gallardo is 27-12 with a 2.48 ERA and 439 strikeouts. He throws a low-90s fastball and a quality curveball, and also can feature a changeup.
"I know him enough to know he's a quiet kid, a confident kid who goes about his business in a real good manner," Yost said. "I think he fits right in with our mature young guys we have here now.
"He's got everything going for him that says he should be successful," Yost said. "But you just have to see it."
The Brewers already called up their top positional prospect. Third baseman Ryan Braun joined the team on May 24 and entered Thursday's game batting .303 with four home runs and 10 RBIs in 17 games.
Adam McCalvy is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.