Now the Edinburgh back row man is now expected to miss the remainder of the season, not only ruling him out of Scotland's Six Nations campaign, but also wrecking hopes he had of impressing Lions coach Ian McGeechan.
Rennie returned to action with Edinburgh in October after missing seven months following an earlier knee operation. But the problem didn't go away and it left the player with no option other than to seek further medical help.
He had an exploratory operation last Tuesday and Edinburgh coach Andy Robinson is ruling him out of his plans for the remainder of the campaign.
"Ross was a key guy for Edinburgh last season and this is a big blow. He has been performing to an impressive level, but he told us he wasn't at the level he wanted to be and couldn't do all the stuff he wanted to," said Blair.
"I'm sure he can come back as good, if not even better, than he was last season."