Former England, Liverpool and Spurs star Redknapp was in Glasgow on Sunday to pay tribute to former Hoops star Stiliyan Petrov.
And he was astonished to see nearly 60,000 fans pack into the stadium for the friendly match in the honour of the former midfielder.
"It was an incredible day," he told the official Celtic website. "There was a pretty amazing atmosphere.
"But I knew it would be like that from being there before and watching games.
"What Stiliyan has been through - he is a credit to himself and his family - and the turnout he got shows how much everyone appreciates what he did for the football club.
"This is about the only place in the world this would have happened. Celtic is an incredible football club.
"I was fortunate to play for a club that's very similar in Liverpool and the fans really respect what you do for them.
"But to turn up here and see 60,000 people, hear them singing You'll Never Walk Alone and have fun as well - I think everyone enjoyed it.
"It was a real family day and obviously for a great cause as well.
"It was very emotional at the end there, and when you have kids it brings it home even more. But everyone turned up and it speaks volumes of how much people respect Stiliyan."