Results of a scan this week have shown that Osbourne has sustained a hamstring tear.
The Aberdeen medical team are hopeful the player will be fit to make a return after the league split, but unsurprisingly the cautious management team would rather not put a timeframe on his recovery.
Dons boss Craig Brown said: "To lose Isaac again to another injury is a huge disappointment for the squad.
"Having just returned to the team in recent weeks, he had made a positive impact on our performances and he will be sadly missed.
"Isaac is a consummate professional and he will work extremely hard to ensure his rehabilitation from this injury is swift."
Source: Aberdeen Mad