Argentina's World Player of the Year Messi and Italy international Balotelli have been sidelined with thigh problems, although Messi featured as a second-half substitute in Barca's 0-0 La Liga draw at Osasuna in Pamplona on Saturday.
"He (Messi) looks in good shape to me because the gamble in taking him to Pamplona was to see how he was feeling," Barca coach Gerardo Martino told a news conference in Milan.
"He recovered more quickly than we expected," added the Argentine, like Messi a native of Rosario. "We felt it was sensible for him to get some playing time."
Before Saturday, Messi had not played since sustaining the thigh injury at the end of last month and missed Barca's 1-0 win at Celtic in their second Group H outing.
He netted a hat-trick in the 4-0 thrashing of Ajax Amsterdam in their opening game and a win for Barca on Tuesday would put them five points clear of Milan on a maximum nine points.
At an earlier news conference, Milan coach Massimiliano Allegri said Balotelli was recovering well after missing Saturday's 1-0 Serie A win against Udinese.
Balotelli was included in the squad to play Barca and Allegri said a decision would be taken on whether he can play after Monday's training session.
Centre back Philippe Mexes and midfielder Nigel de Jong would return to action after resting at the weekend, he added.
"When you play against Barcelona the atmosphere is always exceptional and by now we are regular opponents," Allegri said.
"It is always a special match, the highest level in Europe and they remain the best in the world. We have to be very patient for 100 minutes without conceding anything."
The two European heavyweights have played 17 times in continental competition, with Barca winning seven to Milan's five.
The Catalan giants have had the upper hand in their most recent encounters, knocking the seven-times winners out in the last 16 last season after eliminating them in the quarter-finals in 2011-12.
Barca also beat Milan in the 2005-06 semi-finals on their way to winning the second of their four European crowns.
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