Coles has made five appearances for the Pirates this season
Bristol Rovers fear Danny Coles will have to sit out the rest of the season.
The central defender, 27, had been training with the squad and was on course to make his comeback after five months out with a knee problem.
But swelling in the joint means he now faces another three weeks out and first-team coach Paul Trollope says it is unlikely Coles will play this term.
He told BBC Radio Bristol: "In terms of his rehab time, there's a question mark now whether he'll be back this season."
Coles suffered cartilage damage against Leyton Orient in October and has had two operations on his knee.
But despite the setback, Trollope is confident there will be no lasting damage and Coles will definitely be back in the first-team fray for the 2009/10 season.
He added: "The knee's structurally sound and the repairs that have been done were OK and there was no real further damage.
"But there was some inflammation that needed an injection to settle it down."
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