When three players are crowding one, its not a wise decision to pass the ball to him, even if hes messi (unless hes in his best / goat of goat mode), before 11/12 season.
Even xavi / iniesta would need to find a moment to pass him the ball. Just except it, no one atm has the ability to go around players as iniesta had or keep possesion play like xavi had. Dembele is more like iniesta except hes a winger.
I dont know, if we place dembele close to messi, and there are two other winger coming from the back (remember 4-3-2-1 xmas tree formation of ac milan ?), Would it be better? Although theres some doubt in the defending midfielder. And coutinho could be sacrificed, which i dont really agree.
Or 4-2-3-1 with two full backs ?
Oh and barca's problem is not always main players, the substitutes are actually more of a concern. The level of communication is too low when there are some teams with two set of players of almost equal quality.