Managers United over officials
Villas-Boas claims Webb's decisions changed the course of the game as Chelsea were cruising towards a crucial victory before two penalties were given against them, and said: "The second penalty was very, very dubious."
He added: "I don't know if Howard Webb had the correct angle to make the decision, maybe the linesman made the decision but it was very unlucky from him to give it.
"I'm not sure if he was compensating from anything in the first half, but it was the wrong decision."
United manager Ferguson was thrilled to see his team come back from 3-0 down to steal a valuable point, but he was furious at the performance of linesman Darren Cann, who failed to notice a foul from Gary Cahill on Welbeck after the striker raced through in the first half.
"They should've had a man sent off in the first half," Ferguson said. "Danny Welbeck's clear through and he was brought down. Okay, it's outside the box, (but there was) no decision.
"That linesman, who's given a penalty kick against us from 40 yards away last year against Liverpool, this year against Arsenal - and he can't see that?
"I don't blame Howard Webb - he needed help in that situation and he didn't get it. That assistant referee, who's all too happy to flag at Old Trafford for penalty kicks, didn't give them."
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