The Argentina international forward has been struggling with a hamstring injury in recent weeks and returned to training last Friday.
"In recent days Messi has been feeling good," he told reporters on Monday. "I can't say for sure how he will feel tomorrow. We'll listen to the doctors and physios and make a decision tomorrow. If Leo plays or doesn't play it will make a different but that's because he's a special player.
"This will be a very tight tie. It will be won because of minor details. We'll try to play our very best and do things correctly. It will be a very fast game and very intense. The team that impose themselves on the game with have the advantage. We'll try and impose our rhythm, our pace and our intensity on the game.
"We know how difficult it's going to be. We know how good Bayern are. We know it will be difficult tomorrow and in the tie as a whole. Bayern have been very good in this competition. They are in great form and are a very strong team. We're competitive and we're getting a good feeling back and are really looking forward to it."
Bayern will host the first leg of their semi-final tie at the Allianz Arena in Munich.