The goalkeeper has received criticism for his inconsistent showings of late, including a heroic performance against Borussia Dortmund, mixed with an unimpressive display for his country against Sweden.
"For me he is the first choice but if he does not play well I will change. I don't have any problem [with that]," Mancini told reporters.
"Joe is like Mario Balotelli, like all the other players. For England I think it will be the same. If he does not play well maybe the manager can change the keeper.
"Costel Pantilimon is a good keeper and if Joe does not play well for five or six games, probably I can change the keeper. This is normal.
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"But I trust totally in Joe, for me he is the best keeper in England. I am sure that he will make some mistakes sometimes, this is a keeper's life.
"If a keeper makes a mistake, you concede a goal and you lose. If the striker doesn't score a goal, the game can finish 0-0."
However, Mancini's decision will not be entirely reflective on his mid-week performance.
"Ibrahimovic can score against every keeper," he added. "Ibrahimovic for me is one of the top five strikers in the world."