Nir Bitton is in a race against time to be fit to face Qarabag next week.
The Israeli international, 23, took a while to get going in Scottish football having joined Celtic from Ashdod in the summer of 2013, but he is now considered to be a key player under Ronny Deila, forming a superb partnership with Scott Brown in the heart of midfield.
However, Deila may now have to make plans without Bitton after the defensive midfielder was substituted in the 69th minute of Wednesday's 1-0 first leg win over Qarabag in their Champions League qualifier having suffered a knock to his ankle.
There has been little news surrounding Bitton so far, but The Scottish Sun now report that Bitton missed training on Friday due to his injury - and is fighting to be fit for the return leg in Azerbaijan this coming week, with the player confident of recovering in time.
Do Celtic need another central midfielder?
With Charlie Mulgrew also suffering an injury recently, Bitton's injury shows just how short of options Deila is in midfield, with few players capable of bringing the power and composure that Bitton does.
Tom Rogic could be handed a chance to impress but this could be a message for Deila to make a fifth summer signing in order to strengthen his midfield options.
Another big, strong midfielder like Bitton as cover would be ideal for Celtic, who have so far added a goalkeeper, a right back, a centre back and centre forward this summer - neglecting the midfield to date.