"It's not looking good for Dan," said coach Mark Anscombe. "It's the same leg or calf injury so that's got to be a concern."
The 1999 European champions and last season's finalists are three points clear of Castres going into the final group games but already had inspirational captain Johann Muller (broken arm) and Lewis Stevenson (knee) on a lengthy injury list and now Tuohy is in real danger of being sidelined for the final push for the knock-out stages.
Tuohy had been in impressive form this season, helping Ulster to the top of the PRO12 and to the brink of the Heineken Cup knock-out stages with his last-gasp try at Franklin's Gardens earning the Irish club an impressive bonus point on the road.
It leaves Ulster with Iain Henderson and Neil McComb as their recognised locks and back-up coming from Robbie Diack.
In the same match, centre Luke Marshall suffered damage to a finger and Anscombe added: "Luke is also a real concern as is Jared (Payne) who missed the Scarlets' game."