McCammon, who was signed by The Lions at the end of last season, picked up the injury in the last game of the season when he fell awkwardly after an ariel challenge. Despite intensive treatment it still hasn't healed.
Speaking to Millwall's official website, McGhee commented "Mark has bone bruising, which is where the two bones have clashed and there is damage done between them. It is not a fracture but it is very sore and affects a player's mobility."
The operation will keep McCammon out of The Lions side for up to four months.