A costly error from the England No.1 led to the Saints' second goal on Saturday, as he fumbled a Rickie Lambert shot to allow Steven Davis to bundle home the rebound.
He had also failed to clear in the build-up to Jason Puncheon's ninth-minute opener, allowing the Saints man to stab home.
Roberto Mancini: "Joe has made too many mistakes. He has not been good enough.
"It was a really bad mistake from Joe.
"It was not good enough. It's not strange because he's done a few mistakes this year."