Cameron Carter-Vickers was missing from Celtic training this afternoon ahead of the derby showdown against Rangers.
The USA international, 25, was substituted in the win over Dundee on Boxing Day with a hamstring problem.
It was hoped the knock was nothing serious, however, the centre-back now appears a major doubt after missing training alongside his team-mates at Lennoxtown today.
Brendan Rodgers had initially said of Carter-Vickers after the knock: "Thankfully his hamstring hasn't gone, but he just felt a little bit tight. We've said to him before that if he feels anything, it's better letting us know.
"So he just started to tighten up a little bit, we were able to take him off, and Stephen Welsh comes in and was excellent.
"The medical team will assess him and see where we're at in the coming days."
Celtic did, however, receive a double fitness boost with the return of Reo Hatate and Liel Abada.
The pair were fully involved in training ahead of the match against Rangers on Saturday.
Both players have been sidelined for significant spells but appear ready to return to the first-team fold.
