United going into their clash at Spurs unbeaten in the league and clear at the top of the table.
But Redknapp told the Hornsey Journal: "If you look at United they're not as strong as they were a couple of years ago, they can't be.
"They've lost a player, [Cristiano] Ronaldo, who is just the most amazing player in the world. Along with Messi, he's the best, it's out of those two.
"He's got something like 65 goals in 62 games for Real Madrid. It's beyond belief. So when you take him out of your team you can't be as good. He got 40-odd goals for them one season."
However, Redknapp is not underestimating Sir Alex Ferguson's side, and he believes that their unbeaten league record this season is a testament to their quality and resilience.
"[Dimitar] Berbatov's in great form, he's a fantastic player, and I'm sure [Wayne] Rooney will be back," he said. "When they've got their full team out, with the likes of [Nemanja] Vidic and [Rio] Ferdinand, they're still strong.
"I say it all the time, you need your best players and they've had little spells where Rio's been out or they've had one or two missing - there was no Rooney for a long spell. But they've come through that unbeaten and now they've got their best players back they're going to be even more difficult.