Redknapp, now a pundit on BT Sport's coverage of the competition, believes Spurs pulled off a huge coup signing Alli for just £5m from MK Dons two years ago.
He said: “I think he's fantastic. Can you tell me how all those people failed to spot him when he played for MK Dons every week?
“How did they end up waiting until it took Spurs to go in and take him? He played against Manchester United when they beat them that day.
“United should have signed him then and there. They'd have headed off from the game disappointed, but at least they'd have had a future star for the club with them on the team bus.
“You wonder how they didn't take a kid from MK Dons for £5m. How did they not see him? It's incredible that no-one took him. £5m! What a signing, what a player.
“I think now he's worth £100m. If Pogba's £90m, he's more. He's 20 years of age! The kid's got everything.
“He scores goals, makes goals, can head it, run, dribble, he's got an edge to him too and he puts his foot in. I think he's fantastic."