The 27-year-old was injured May 1 at San Diego when he hit a wall while chasing Nick Hundley's foul popup, cutting short his season after 21 games. Gamel reinjured the knee Saturday during Milwaukee's first full-squad workout this year.
"It really surprised me because there was no really major event that happened that you would have thought that would have been that serious," Brewers manager Ron Roenicke said. "I figured, OK, he tweaks it. Coming back after a rehab you figure it is not always going to go smooth. I figured a couple of days and he'd be back on the field."
An MRI Sunday revealed the extent of the injury.
"It's a tear of the middle portion of the repair, "There are failures -- 10 percent seems to be the historical number. But of those failures, they're mostly at either end" of the ligament.
Ash said Gamel's knee checked out fine in late January in Milwaukee and again during a spring training physical on Feb. 12.
Corey Hart, who moved to first base after Gamel's injury last year, is on crutches after right knee surgery on Jan. 25 and is expected to be out three to four months.
"We'll look at all the guys we have in camp here," general manager Doug Melvin said. "We'll look internally."
Gonzalez has never played first in his 14 years in the majors and does not have a first base glove, so he borrowed one.
"Ron talked to me about it, but it's not sure yet," Gonzalez said. "I don't want to tell you, 'Yeah, I'm going to be the everyday first baseman.' I'm just going to wait and see what happens and make a decision."
Crosby, the 2004 AL Rookie of the Year, has not played the past two seasons and has only 31 games at first in the major leagues.
Morris was the Brewers' minor league player of the year last season, hitting .303 with 28 home runs and 113 RBIs with Huntsville. He has never played above Double-A.
"It's a big jump," Morris said. "Us talking about it doesn't mean I'm going to get the job. I've got to earn it. I've got to do a lot of things extraordinary. I feel terrible for Mat. Opportunities like this rise all the time in this game. It's up to me to take advantage of it.
Green played 18 games at first base last season with Milwaukee but hit only .184 in 104 at-bats. Hatton batted .274 with 17 home runs and 57 RBIs last season with Triple-A Nashville.
"I think we can cover it with people we have in-house," Roenicke said. "But that's not to say that Doug isn't already thinking about it and looking, because you can't predict exactly when Corey's coming back.
No surgery date has been scheduled yet for Gamel.
"He was pretty good when I talked to him earlier today, but it has to crush you," Roenicke said. "You go through a long rehab period, which isn't fun. It's lonely, it's hard work, boring at times, and now to think you have to go through it all again, it's pretty rough."
Gamel has been restricted to 106 games in the majors since his debut in 2008, mostly because of injuries, including a torn muscle in his right shoulder in March 2010 and a strained his right oblique in February 2011.
"Clearly it's a setback," Ash said. "It's a tough thing just as much mentally as physically for a guy that's spent eight months rehabbing and all of a sudden be told he'll have to start over again. ... He does have time on his side, but clearly missing that much repetition in games is going to hurt him."