Casillas has cranked up the pressure on Roberto Mancini by insisting the City boss has to deliver this season.
He told the Daily Star Sunday: "Like so many of City's players, Hart needs to play Champions League football. If he doesn't get it, he won't stay.
"I think it's fine if you want to go and be part of the revolution at City.
"But I would suggest all players write clauses into their contract about being able to leave the club if they don't make it into the Champions League."