I actually see it from a different side, Messi presence in the Barcelona squad will further improve the quality of Barcelona's game. Messi will take on a new role this time around, he will be shifted to the center to serve the attackers with his beautiful cross. He can also set the rhythm of the game well, the 4-3-3 formation can still be used with the current attacking scheme, Messi will fill in as an attacking midfielder.
I've watched Messi a lot to know this won't work. Messi likes to float to the middle, but it doesn't mean he likes being played in the middle. He performs best when he is in his preferred right-wing, that way he can float to the center and even to the striker position. He's no longer a young man so he has to play according to his strength. Hes too old to struggle for 90 minutes in the midfield.
I believe Messi will be a wonderful addition to Barcelona too and young players will benefit from him, but every player on that right wing of Barcelona would be sacrificed except Dembele that can actually adapt to the left wing. Lewnadoski too will be one of the biggest beneficiaries of Messi's return.