Oster damaged his cruciate and medial ligaments while playing for Sunderland reserves, and will be out of action for the rest of the season.
Oster left The Mariners to join Everton in 1997, and Groves was on the verge of bringing him back to the First Division strugglers before his injury.
Groves was set to talk to Oster's agent over a return to the club, and was gutted that the injury scuppered the move.
"We were going down that route but John got injured and there's nothing we can do but wish him all the best," said Groves.
