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He added: "The injury flared up the day after the game. We felt it was only a slight new problem where he had extended the joint after slipping.
"Our medical staff, when a player reports an injury the following day, reviewed the video to try to find out where the injury occurred.
"The only place we could see where it happened is when he tried to change direction and he slipped on the surface, so over extending the joint and it split the cartilage slightly.
"There was no point in messing about. It was straight in for the operation and it was done and dusted by Tuesday.
"Unfortunately, he is going to be missing for four to six weeks and it is a huge loss for us."
Allardyce could be without up to eight players for the game against Arsenal at Highbury on Saturday and will conduct a series of fitness tests before naming his side.