He needs to stop fucking wandering so deep to get the ball and be patient and wait til someone finds him in space in the final third. If he can't find space centrally because there are sixteen other 10's occupying that space he should pull wide or as a senior player tell them to position themselves wide.
He hasn't lost much physically, he can still finish, he can still play a pass, he still has great vision and is still excellent at linking up with team mates. He just doesn't have the tactical discipline to play in a system because he fucks the system up when he goes wandering.
EDIT: And IMO the system is as important as the players you have in the team. You can have 11 Messi's in your team but the team wouldn't win you the champions league
Messi would be quite the shitty keeper.