But although scans have shown a fracture in his shoulder, Vuna is only expected to be sidelined for six weeks, ruling him out of the opening four matches of the Super XV campaign but leaving him plenty of time to pres his claims for a spot against the Lions.
"Initially I predicted the worst, so to see myself coming back into contention early on in the season is a real relief," said Vuna.
"I'll be working hard to be back firing ready to go for my teammates."
Vuna was only just returning to action after a lengthy absence having opted to have reconstructive surgery on his other shoulder in August in order to be fit for the start of the new season.
That decision had cost the 25-year-old a place in the Wallabies' Tri Nations squad after he had won his first two caps against Scotland and Wales in June, making the latest problem even tougher to take.
During the pre-season he dropped a staggering 10 kilograms to get down to 102kg and was flying in training and although the fractured scapula won't require surgery, it's still a setback.
"I've done everything in the pre-season and done everything right with my shoulder rehab so to do the other shoulder, it's not ideal," added Vuna, who was in superb shape for the new year after dropping 10kg in pre-season.
"I was in good shape and where I wanted to be in terms of training so I'll have to start all over again.
"Next week I'll meet with the conditioning coach and try to get on top of it again.
"It could be worse but it's still a long time."
Vuna has previously missed just three of the Rebels' 32 Super Rugby games after their entry into the competition two years ago and was the side's leading try scorer in both 2011 and 2012.