Having taken a shock - but deserved - 2-0 lead at half-time, Redknapp's side were on course for a win that would have blown open the title race with second-placed Liverpool having already triumphed at Hull earlier in the day.
But a controversial penalty awarded by Webb - who only last week was announced as next month's FA Cup final official - early in the second-half for an alleged foul by goalkeeper Heurelho Gomes on Michael Carrick reignited the United players and supporters, allowing them to run riot as Spurs wilted under the pressure.
"The players can't believe it. It changed the game," said Redknapp.
"The referee made a terrible mistake. You should have technology. The fourth official should be watching the TV monitor and talking to the referee.
"United were not going to win unless something like that happened.
"The referee has not done it on purpose, it is just a bad decision but it was the turning point."
United boss Sir Alex Ferguson, unsurprisingly, was happy to take advantage of Webb's gaffe, especially after his side were denied what appeared a spot-kick in last weekend's FA Cup semi-final against Everton at Wembley.
"We were a bit fortunate but football is a funny game," said Ferguson.
"You get terrible decisions. We got one last week. You have to deal with them and get on with it."
NEWS: Webb admits to penalty gaffe
NEWS: Jenas: Webb wilted under Old Trafford pressure
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