They don't play with an out and out #10 in the formation but it's been said that they need a player of that style/profile. Koke isn't a dribbler and Arda isn't a straight up playmaker, but Diego is both.
Diego can play in the wide areas well anyway, maintaining his style of play. Koke is being shifted to CM more often recently and performing very well there, so Arda and Diego as the wide AM's means that all 3 can stay with ease. Not to mention they have Tiago, Oliver, Sosa, etc. to rotate often with. None of the 3 has to leave next season, and it's looking less likely that any of them will. If any player will leave it's Costa and that too is debatable considering his commitment.
Insua isn't the best of left backs though, he'll work while Luis is out right now. Up front is a different ballgame as right now they seem ok but you never know. If they lose Costa, they have no one to rely on soon. Baptistao can develop in a few years (he's doing admirably at Betis, just missing the goals) but Villa is aging and Raul Garcia isn't a go-to man even if he functions. Correa would be a good investment since he has the potential to be among the strikers that Atletico develops to a very high standard.