Ash said it was determined the only course of action was surgery.
While the meniscal tear also will require rehab, Ash said recovering from the debridement -- which is the removal of lacerated, dead, or contaminated tissue -- is the biggest factor.
"When you have that kind of surgery, you can't be weight bearing for up to six weeks," Ash said. "That's really what slows down the recovery."
Although Hart is expected to miss games during the season's first month or two, Ash said he isn't concerned about Hart coming back. After all, the outfielder-turned-first baseman has suffered numerous injuries in his career and has returned fine, and in a timely fashion, each time.
"He's had a terrific track record coming back from injuries. He's usually beaten the forecast, in terms of time," Ash said. "Some of this is about the surgery, and some of it is the effort you put into recovery, and he's always been a good rehab patient and follows the program. Track record-wise, he'll probably be back sooner rather than later."
Hart's injury now reopens first base, at least for the time being, to Mat Gamel. Gamel, 27, was Milwaukee's Opening Day first baseman a year ago before suffering a torn anterior cruciate ligament in his right knee in early May.
Ash said trainers and therapists have reported that Gamel is "100 percent, physically."
"He's gone to rehab all winter long and he is 100 percent," Ash said. "The only thing he hasn't done is gotten on the field and participated in games. That part we don't know, but physically, he's fine."
Gamel hit .246 with one home run in 21 games last season, while Hart batted .270 with 30 homers and 83 RBIs in 149 games.