Ulster back row Nick Williams looks likely to miss Saturday's European Champions Cup game against Saracens.
Williams' half-time introduction helped Ulster earn a dramatic 24-23 victory over Oyonnax on Sunday, but he took a blow to the head late in the game.
The 32-year-old will be assessed this week under the normal concussion protocols but the expectation is he will miss the Pool 1 contest.
Centres Jared Payne and Darren Cave could return this weekend.
Payne has been out since breaking a foot during Ireland's second World Cup pool match in late September.
Fellow Ireland international Cave has been missing since early December because of a shoulder injury.
Ulster's long-term injuries include Iain Henderson, Tommy Bowe and Peter Nelson.
Ireland star Henderson (hamstring tear) and Nelson (broken foot) and both out for the rest of this season while Bowe will be missing until at least the latter part of the campaign because of a knee injury.
Centre Stuart Olding is nearing a return to fitness after being out since March because of a knee injury while another Ireland player - Dan Tuohy - is expected to be available again next month after breaking a leg in early December.
Williams, who will join Cardiff Blues in the summer, had to be replaced a minute before the end of Sunday's game in France and blood could be seen coming from a head wound after the final whistle.
Meanwhile, Ulster director of rugby Les Kiss has defended his selection for Sunday's game after his side had to fight back from 23-0 down at half-time.
Kiss left regular half-backs Ruan Pienaar and Paddy Jackson out of the starting line-up but, after a dismal first-half display, the duo helped the Irish province fight back to snatch a dramatic win.
"We don't pick a team flippantly," Kiss told BBC Sport Northern Ireland, who added Sunday's first-half struggle was a "team failure".
"We go through a lot of variables. You are looking at players that you need to have a bit of rugby under their belt.
"With the Six Nations coming up, we need to have the like of Ian Humphreys playing a bit of footie before then and you have got to look at the players across the board and manage that right."