While moving Messi to the wing is one plan, another is that the coaches could work with Messi in practice to make him better at playing the forward role for games like this. Here's one idea I have: in practices for the Milan game, put cones on the sidelines about 30 meters from the "opponent's" goal, and tell Messi that he should never go closer to his goal than that in open play, unless doing so would put him in an offside position.
Last year against Chelsea, when he lost the ball that led to Drogba's goal, I wasn't annoyed with the dispossession as much as the fact that he was behind most of the midfield when he was nominally the central attacker. Against Milan, he again sometimes dropped deeper than Busquets. Against weak La Liga sides we get away with it, but against elite opponents that simply cannot happen. The biggest weakness to Messi's game is tactical indiscipline. I find it hilarious how after some games (Chelsea, Celtic, Milan) in which Barcelona dominates possession but doesn't score and Messi drops too deep, some people say that it's okay because "he was trying to make something happen" and the midfield "wasn't getting the ball to him." (Welcome to what most teams feel like.) Right now, teams know that if they cover their defensive third well, Messi will take himself out of dangerous positions to get the ball. This brings me back to what I said about working with Messi in practice on playing like a traditional number nine. I don't mean winning headers; I mean making runs and evading centerbacks the way Eto'o did. In fact, someone on the Barcelona staff could just make an Eto'o video, send it to Messi, and say "learn."