Vuna got injured in the pre-season friendly against the Waratahs at the weekend, coming off the field after only 18 minutes in the 24-16 defeat at the North Hobart Oval.
The injury was initially thought to be just a 'stinger', but the player is set to undergo X-Rays to reveal the true extend of the damage.
"We're not sure what's wrong. He's sore behind the shoulder in the scapula area so obviously we'll just wait and see how it settles down and the doctors will make a decision on that," said Rebels assistant coach John Muggleton.
But while Vuna's injury was a blow to the Rebels, they were boosted by the return after a lengthy injury lay off of utility back James O'Connor, who made it through the whole game.
"The last three or four times he's played he's been injured so for him to get through that ... was good for his mental approach," said Muggleton.