He spent a period at Celtic early in his career before making 357 appearances for Newcastle, and Celtic manager Neil Lennon has confirmed he would like to sign the Manchester City goalkeeper on loan.
Lennon: "He's one of the best keepers in the Premier League.
"We've been linked with a lot of players and there's been speculation on a lot of players.
"I don't really want to comment on players coming in or going out. I'm just concentrating on the game on Thursday against Utrecht."