Neil Lennon fears Adam Matthews could be out for three months after injuring his collarbone in the 2-0 win over Motherwell at Parkhead which took the Hoops top of the Scottish Premiership.
Goals by Anthony Stokes and Kris Commons gave the home side a comfortable win but it was marred by the injury to their wide-man who was carried off on a stretcher before the break after a hefty challenge by Well skipper Keith Lasley.
Matthews will miss Wales' World Cup qualifying double-header against Macedonia and Belgium and if the diagnosis proves Lennon correct, then he will miss the rest of Celtic's Champions League group games.
The Hoops boss said: "Our initial fears is it could be a fractured collarbone.
"If that is the case we could be looking at three months. We don't know yet, we haven't had any word back from the hospital yet, that's where is at the minute.
"If that is the case then it puts a big negative on what is a great day for us."
Lennon believes Celtic should have got a penalty for Lasley's challenge on Matthews, but did not blame the Fir Park captain for the injury.
The Northern Irishman said: "I thought it was a penalty. He has taken his leg first and followed through on the ball - anywhere else that is a free-kick.
"Adam did the right thing, he got his body there protecting the ball. For me it is a penalty.
"It was maybe the way he landed. It could happen anywhere.