Mancini faces an uncertain summer after falling short in the Premier League and the Champions League, with Monaco said to be keen to offer him an escape from The Etihad.
Mills told the Daily Star Sunday: "It is a simple fact that City have fallen a long way behind United.
"Mancini has said if he's under pressure then every other manager deserves to be sacked.
"But that's not the whole truth because it's not a level playing field.
"They've spent fortunes. There's been no real improvement this season. If anything they've stood still and maybe even gone backwards."