The Hammers had taken the lead at Old Trafford through a stunning Dimitri Payet free kick but during the closing stages, Anthony Martial struck from close range to ensure that the two teams will meet again in a replay at Upton Park.
However, Webb feels that official Martin Atkinson should have chalked off the goal after Bastian Schweinsteiger collided with goalkeeper Darren Randolph, preventing the Republic of Ireland international from trying to get across his goal in order to keep out the effort.
It's questionable whether Randolph would have got in the way of the ball before it crossed the line, but Webb insists it is irrelevant.
Webb said: "When the cross comes in, you see Schweinsteiger step into the path of Randolph. Whether it is careless or intentional, it doesn't really matter - it's a foul.
"It's such a difficult decision for Martin Atkinson to make in a crowded penalty area but he puts his body into Randolph and stops him from getting across.
"Would he have stopped the shot? Maybe not but it's a foul and if Martin sees that, he disallows the goal."
Atkinson's decision was criticised by West Ham co-chairman David Gold, but the draw has ensured that West Ham can play one last FA Cup fixture at Upton Park before they move to the Olympic Stadium in the summer.
The winner of the replay will join Crystal Palace, Everton and Watford in the last four of the competition.