The New Zealand international  was forced off with the problem during last weekend's 1-1 Barclays Premier League draw against Birmingham and manager Steve Kean revealed there was no option but for Nelsen to go under the knife.

The centre-back partnership between Nelsen and captain Christopher Samba has been one of Blackburn's main strengths and the news is a major blow to their survival hopes.

Kean said of the operation: "It went well. The surgeon was happy about it and he had a full clearout.

"There were a lot of little bits of floating debris in his knee that he's been in a lot of pain with. He couldn't manage to go on. We're hopeful he'll be back in time for pre-season."

With Rovers only three points above the relegation zone heading into Saturday's trip to Everton, Kean can ill afford such setbacks, but he cited the return of youngster Phil Jones from his own knee injury as a saving grace.

"Fortunately for us we lose Ryan at a time when Phil Jones is back fully fit," added the manager on www.rovers.co.uk.

"He's slotted back in there and looked like he'd never been away from central defence, so that is fantastic for us, but it is unfortunate that we lose Nelly."