United - trailing 2-0 to Spurs at half-time - were handed a lifeline early in the second period after Webb adjudged goalkeeper Heurelho Gomes to have fouled Michael Carrick inside the danger-zone, with replays showing the glove-man clearly won the ball.
Cristiano Ronaldo despatched the spot-kick, with the Premier League holders going on to run riot against their flabbergasted opponents, scoring another four to complete the unlikeliest of 5-2 victories.
And Jenas believes the match official's dubious decision was a result of him buckling under the weight of a vociferous crowd at the Theatre of Dreams.
"That was a prime example of a referee crumbling over the pressure at Old Trafford," he said in the Mirror.
"I don't want to make excuses and we went on to concede another four after that, which were sloppy goals, but it would have been a completely different game had they not got that first goal.
"It's just the way football is sometimes and pressure gets to people. The atmosphere, the occasion, the importance of the match, a lot of factors take their toll when making decisions.
"One thing that struck me about it was that he didn't even think.
"It was like he'd already made his mind up when he came out for the second-half that he was going to give something."
NEWS: Webb admits to penalty gaffe
NEWS: Redknapp rages at Webb spot-kick shambles
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