The pair didn't see eye-to-eye when Ibra had a season at the Nou Camp.
"Today, for sure Leo Messi's the best player," Ibra said. "Maybe they should put the name Messi on the Golden Ball, he has a couple of years more to win it.
"As an active player, it's difficult to say he's the best who's ever played football. Messi has to quit football, then you can decide if he's the best or not.
"It will be a difficult game, they are probably the best team today and probably the best team ever playing football on this planet.
"It will be very exciting to try to find how to play, how to disturb them. Milan played a good tactical game and disturbed Barcelona in their game."
Former England captain David Beckham will be available and Ibrahimovic feels he has a major part to play.
"He has big quality and he's a fighter, he has a big heart," said the Swede.
"With two months left, we are still playing for so many competitions and that's where he becomes so important, not all our players have this experience of winning trophies."