The Italy international missed Saturday's 1-0 victory over Udinese due to a thigh injury, while his status for Tuesday's match remains in question. Allegri refused to rule out the possibility of the 23-year-old being fit to feature against the Blaugrana, explaining that he will wait closer to kickoff before deciding whether the forward will play.
"Balotelli is doing better," Allegri told reporters. "We'll check on his condition in training today, and we'll see if he can be called up. The physical condition of the side is improving. We all have to work in the same direction."
Despite a growing list of injuries, Allegri believes his team is capable of repeating its 2-0 home victory over the Catalan giants in last season's round of 16 first-leg clash.
"It will be a fascinating game for everybody," Allegri said. "We need to play with courage and sacrifice against the best team in the world to get three points and increase our chances of qualifying.
"I believe that tomorrow Milan will perform well, we must try to make as few mistakes as possible. Barcelona will control the ball more than us, so we have to play a game similar to last year's."
Milan sits second in its group with four points from two matches, while Barcelona has a perfect record.