Dragons have suffered a major setback after in-form fly-half Dorian Jones was ruled out for a few months with a chest muscle injury.
Jones, 23, was forced from the action against Ospreys with the problem and director of rugby Lyn Jones admits it's a blow for his charges.
"Dorian has had a pectoral injury, which looks quite serious," he told BBC Sport.
"It could be a few months, which is a blow to us because he's right on top of his game, starting to challenge for national honours behind Dan Biggar."
Also joining Jones on the sidelines on New Year's Day was scrum-half Sarel Pretorius, who suffered a calf injury. However, the Dragons boss revealed that their absences are set to be cushioned by the returns of Wales duo Hallam Amos and Tyler Morgan later this month.
"They are big-time players who can offer something different," added Jones.
"There's a good chance one of them will be back [to face Castres], which one I'm not sure.
"They've made really good progress and the medical department need to be congratulated for getting them back with such speed.
"They've recovered very well and we expected both of them to be playing some part in the European competition shortly."