The FA Cup may offer City's best route to silverware this season but Marsh maintains Mancini is doomed.
He told the Sunday Mirror: "Mancini is a terrific manager. "I like his attitude. But he will be let go even if we win the FA Cup. I would prefer them to keep him than let him go. But he has been very poor in the Champions League. Some of his signings have not been of a standard that nowadays is identified as Manchester City quality.
"As far as the league is concerned it is over. This is partly because of the great distraction with Mario Balotelli, who let the team down when he was there at the beginning of the season. But it's also because Mancini hasn't at times seemed to pick the right balance of players.
"He hasn't been helped by injuries to key players, especially Vincent Kompany.
"Kompany is the captain and was fantastic last year. But he's missed so many games this season and this has had such a detrimental effect on the defence - in fact the whole team.
"Balotelli was a nightmare and the saga with him also had a major effect on the team's bid for the title."