Well of course a manager is going to be more successful with a player like Messi than without them.
It's like saying would Wenger have been successful if he didn't have the likes of Henry/Viera and co saving them. Or would Jose have been successful at Chelsea if he didn't have Robben/Terry/Drogba/Carvalho in 2005 or Hazard from 13/15.
Although it does go to show, how much of a teams performance is down to the manager and how much is down to the innate ability of the players.