The former Hoops hitman, speaking exclusively to Football Insider, has been blown away by the stopper’s start to life at Parkhead.
Hart, 34, signed for Celtic from Tottenham in the summer and he was immediately installed as the club’s number one.
He has already made 18 appearances for the club with the latest coming in Saturday’s 2-0 win over St Johnstone.
This result gave Hart his third clean sheet in as many games with the Hoops defence looking to have found some solidity.
McAvennie continues to be wowed by the Englishman who was “down and out” before joining the club.
“Joe Hart, I mean, wow,” McAvennie told Football Insider correspondent Ben Wild.
“He was down and out before he came to us, I know he was playing a bit at Tottenham but it wasn’t going well.
“That came after all the problems with Man City and the loans he had. I think that made him a stronger character.
“Obviously, I don’t know him so I’m just guessing but it looks like he’s enjoying himself here.
“I said when he signed I thought it was a good move. We haven’t heard from the other two lads for a while, have we?
“Hart has barely put a foot wrong, he’s been incredible. There has been one mistake in Europe but you can forgive that for how well he’s done in all the other games.
“He’s been a great signing. He has a big role to play and he does it so well.
He is a much-needed voice in that defence and it speaks volumes that he’s already captained us.”