Friedel is believed to have damaged the cartilage and not the ligament as was first feared, and he told BBC Radio Lancashire: "I am having an operation on Sunday night after the Everton match.
"I have a chance to may be only be out for one game at Charlton.
"I don't feel comfortable in day-to-day life or playing at the moment. It is not very nice when you can't straighten out your leg, so I am going to get it done."