Celtic manager Neil Lennon admits Gary Hooper could have played his last game for the club.
Hooper, who was the subject of bids from Norwich and Hull earlier this year, has a year left on his contract and wants to return to England.
The 25-year-old, whose pre-season has been disrupted by a groin strain, did not feature in Celtic's 3-0 defeat by Union Berlin on Friday night - their fourth friendly loss in Germany ahead of Wednesday's Champions League qualifier against Cliftonville.
Lennon told several national newspapers: "It will be difficult to convince Gary to stay. He has not featured much in pre-season and the games start in earnest for us on Wednesday night.
"So he'd have a bit of catching up to do and I don't know where his mind is at the minute.
"Has he played his last game for Celtic? Never say never but it's looking more and more that way."
Lennon lost Victor Wanyama to Southampton in midweek and believes he needs further reinforcements after bringing in Dutch defender Virgil van Dijk and Portuguese striker Amido Balde.
He said: "I haven't spoken to Peter Lawwell yet about bringing someone in but we need two or three players if we're going to get back into the Champions League."