Shunsuke Nakamura may miss Celtics final two group games after scans on his injured knee reveal more damage.
The Japanese playmaker is currently walking with the aid of two crutches and has a brace on the left knee which has plagued him all season.
Originally the problem was diagnosed as an injury to the ligament on the outside of his knee, the result of a heavy challenge during the Champions League qualifier against Spartak Moscow in August.
Now, after another MRI scan on Monday, Nakamura fears he may have suffered damage to the nerve next to the ligament.
Nakamura said: "I have been told by the doctors to rest as much as possible as that will help me to recover completely from this injury."
Nakamura admits he does not know how many more games he must miss. But it is a huge blow to the player and to Celtic.
Celtic have a fortnight of monumental proportions, with tough away games V Milan and Hearts to look forward to plus the must win match against Shakhtar a week today.
They return to action against Aberdeen on Saturday, then face Shakhtar on Wednesday in a game they must win to have any chance of qualifying for the last 16.
The following week they travel to Hearts, and three days later take on Milan in the San Siro.
The injury has already forced Naka to miss the games against Benfica, along with a handful of domestic games. He last played in the SPL win over Motherwell on October 27. Since then he has been resting the knee, but continuing to work on his upper body.
Get well soon Naka-san
Yours in Celtic
Clydebuilt