Messi has never enjoyed a good match against talented, deep defending/counter-based tactics.
Tello should start and play his wide left role. Messi should be in a center/right role. Alves should take his opportunity to cut inside as Messi moves wide, and look to unleash is patented long shot. Messi can't only stay in the middle as he will be forced to drop deeper and deeper (great news for defenders). Let Xaviesta run the midfield. Start Villa at center forward, and let him pull Milan out of shape with his movement. Make Pedro first off the bench, or make him the outside left.
Barca needs proper structure (see second half vs. Sevilla).