The 27-year-old has been out since sustaining ligament damage in the Pirates' 2-1 home win over Leyton Orient in mid-October.
After two weeks back in training with the rest of the squad, Coles was due to make his comeback in a behind-closed-doors game at Hereford on
Wednesday - but was forced to pull out after his knee swelled up.
Rovers' physio Phil Kite said: "Danny had been back in training without any problem but late last week his knee flared up again. He has since seen a specialist and has had an injection and now we will give it a couple of weeks to settle down and see how things go from there.
"It is a setback for Danny but not a disaster. The specialist said that it is quite common to get these sort of complications after the injury he has had and it doesn't look as though he will need more surgery.
"Whether he can play again this season, we will have to wait and see."