Celtic captain Scott Brown was carried off on a stretcher with a suspected hamstring injury as his side drew 1-1 with Rapid Vienna in a friendly.
The midfielder pulled up in the first minute after an innocuous tackle as he controlled a bouncing ball.
The 29-year-old Scotland skipper had his right knee strapped into the stretcher as he left the field.
Celtic later tweeted: "The captain is being assessed by medical staff and the extent of the injury is not yet known."
And manager Ronny Deila told the club's website: "We think it was his hamstring, but we don't know how severe the injury is.
"Hopefully it's not big and he will be back quickly. He will have a scan tomorrow and we will see what happens."
Teemu Pukki equalised for Celtic after Virgil van Dijk headed into his own net in the seventh minute.
The Bhoys begin their Champions League qualification campaign against KR Reykjavik in Iceland on 15 July.