Beram Kayal will be out for six to seven weeks with a broken bone in his foot, according to Celtic manager Neil Lennon.
The Israel midfielder hobbled off after 30 minutes of the 3-0 UEFA Champions League defeat to Milan at Parkhead on Tuesday night.
Ahead of the Scottish Cup fourth-round clash with Hearts at Tynecastle on Sunday, Lennon said: "Beram has broken a bone in his foot so he is going for a CT scan on Friday to see if it requires a screw putting in.
"He will be out for six or seven weeks with that."
Lennon will be consoled by the fact that both Scott Brown and Nir Biton are both available this weekend, while Joe Ledley played the last hour against Milan.