But the United boss said:" I can't remember going to the Lane with Tottenham challenging for the title.
"That's a measure of their change under Harry. He's done a fantastic job. They have some really exciting players now."
Tottenham have won a host of plaudits this season for attractive football with players such as Rafael van der Vaart and Gareth Bale propelling them to the last 16 of the Champions League while they also sit eight points behind table-topping United.
Ferguson's team, meanwhile, have been widely considered to be playing well outside their full potential, despite being the top scorers in the league and enjoying a two-point lead with games in hand on all their rivals.
Redknapp, however, suggested it was only a matter of time before they hit top form, adding: "We've seen some fantastic Manchester United teams the last few years, the stuff they've played has been incredible.
"I don't think they've hit the heights this year, in terms of playing as well but they keep winning games - that's a fantastic achievement.
"Sooner or later they're going to hit their top form, and then everybody's going to be in real trouble."
ALISTAIR MAGOWAN BLOG Teams that have had success against Tottenham this season have generally been able to nullify the threat of Gareth Bale and Rafael van der Vaart Redknapp has questioned whether United can emulate the Arsenal "Invincibles" side of 2004, who went all the way through the season without losing while Ferguson admits a blip is almost inevitable.
Asked about Tottenham's chances of being the team that ends United's run on Sunday, Redknapp suggested the other title contenders would be praying for a Spurs win against a team who have drawn seven of their nine away games so far this season.
"It would open it [the title chase] up again," he said.
"At the moment they're in fantastic shape and have got a good lead over everybody at the top, so everybody will be hoping we can pull it off and close the gap to keep everybody involved.
"It's not easy, but we're playing well and we've been in great form. It's a great game to look forward to. We're a better team than last year so we've got every chance. We'll give them a real game."
United are set to welcome back striker Wayne Rooney and centre-back Nemanja Vidic after the pair sat out last weekend's FA Cup win over Liverpool, while midfielder Paul Scholes may also return.
Tottenham remain without defender Younes Kaboul, who is completing a three-match ban.