The American performed heroics during Rovers` 2-1 win over Arsenal on Saturday, and Souness has revealed that he was lucky to play in the game at all.
During the warm-up, Friedel`s knee cartilage 'popped out', prompting the Rovers physios to carry out a patch-up job.
Friedel performed heroics to keep The Gunners at bay, but he will have to give in to the injury at some point.
Souness said: "He has a cartilage problem, hence the strapping on his knee which he has worn for a few weeks.
"He will need an operation, but only when Brad lets us know he is ready."
With the busy Christmas period looming, Souness added: "We need to find the right time to operate."
