Wing Peter Hynes has been ruled out of the Reds' Super 14 match against the Crusaders in Christchurch on Friday.
He is out with fluid on his knee and will have a scan to determine whether surgery is required.
His absence comes amid mounting injury concerns for the Reds, who have already lost scrum-half Will Genia for three to four months following an operation on a finger tendon injury sustained - along with rib cartilage damage - in the 52-13 thrashing at the hands of the Brumbies last time.
Flanker Hugh McMeniman is also expected to be out of action for up to six months after after he was forced to withdraw before the Brumbies game with an ankle tendon injury sustained in training.
The Reds also have doubts over five other players, including captain James Horwill, openside flanker Poutasi Luafutu and back-line general Berrick Barnes, who suffered a groin injury during the pre-match warm-up against the Brumbies.
Meanwhile, league convert Mark McLinden has a medial ligament complaint and Digby Ioane is nursing a sore shoulder.
"They're in until they're out," was all head coach Phil Mooney could confidently say about the quintet.
He was originally planning to announce his team on Wednesday but said he now may not know which players he can take to New Zealand until Wednesday evening, the night before they fly out.
Barnes, McLinden, Horwill and Ioane will all be given more time to respond to treatment, while Luafutu has told Mooney he is prepared to play, despite sustaining a fractured hand against the Brumbies.
The head coach has already put Reds Academy scrum-half Brendan McKibbin on notice that his services will be required in Christchurch.
If other key players are ruled out, he will need to delve further into the Academy ranks, with Australian Sevens scrum-half and fly-half Richard Kingi another likely prospect for a call-up.