Ospreys are sweating on the availability of Wales loose-head prop Gareth Thomas.
The front-rower was given a down week when his teammates were preparing to face Montauban in the Challenge Cup after his autumn efforts.
However, he suffered a calf niggle in training and sat out last weekend’s URC loss to Munster in Bridgend.
“He is struggling a little bit and we are tracking how he is going,” said head coach Mark Jones.
“We are picking from a fairly healthy squad, which is positive. We will need the squad through this period because the games are physical by nature because they are derbies.”
Scarlets and Ospreys name their teams at midday on Christmas Eve.