"I think he's pulled him, got his arm on his shoulder. It's not so, so blatant, but he's through on goal and it's put him off balance.
"I think the young lad's arms go up immediately after he's tugged him and I think that body language tells you an awful lot."
Asked if he thought it was because the game was at Old Trafford that the penalty was not awarded, Allardyce replied: "Absolutely".
However, United boss Sir Alex Ferguson backed Webb's decision.
"The boy Pedersen dived," said Ferguson. "There is no question about that at all."
But Ferguson did question Webb's decision to book Cristiano Ronaldo earlier in the game for diving and not Pedersen.
"Both cases were the same," said Ferguson. "You have to give either a penalty or a yellow card."
Ronaldo could have seen red had Webb chosen to punish him for kicking out at David Dunn during the second half, but Ferguson claimed the incident had been overblown.
Pedersen bemused by decision
"The ref spoke to him when he flicked out," he said. "Ronaldo says the boy kicked him. The ref gave him a warning.
"Because it's Ronaldo you're making a meal of it. It's one of those things. He has to live with that kind of examination."
Ferguson also took issue with Webb over the decision not to allow a Jonny Evans headed goal to stand, though the Scot admitted it came from a corner that should not have been awarded in the first place.
"There is no question (it was a goal)," he said. "I think sometimes the refs give a free-kick when they have doubts about a corner.
"It was either a free-kick (to us) or a goal kick in the build-up, and maybe that's why he had a doubt about it. But it was a clear goal."
The win sent United eight points clear at the top of the table, though Ferguson's side have played a game more than Liverpool.
"Sometimes when you don't play your very best you can get a result like that and it goes some way to winning the title, hopefully," said Ferguson.
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