Celtic manager Neil Lennon has admitted striker Gary Hooper is likely to leave the club this summer.
The 25-year-old, who has scored 82 goals since moving to Glasgow in 2010, has one year remaining on his contract and has turned down an extended deal.
have both had bids for the forward rejected this year.
"It will be difficult to convince Gary to stay," Lennon told reporters following the Scottish champions' 3-0 friendly defeat by Union Berlin.
"He has not featured much in pre-season and the games start in earnest for us on Wednesday night [against Cliftonville in the Champions League qualifiers].
Lennon watches on during the friendly in Berlin
"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."
Hooper did not feature in Friday's match in Germany as Lennon's side suffered a fourth successive pre-season loss.
The former Southend and Scunthorpe forward has been Celtic's top scorer in each of the last three seasons.
During that time, he has helped the club win two Scottish Premier League titles, two Scottish Cups and, in 2012-13, reach the last 16 of the Champions League.
The most recent bid for Hooper, an offer of around £4.5m from Hull, was short of Celtic's valuation of the player.
Victor Wanyama left Celtic to join Southampton on Thursday in a £12.5m deal
- a record fee paid for a player from Scottish football.
Lennon has signed
Dutch defender Virgil van Dijk
Portuguese striker Amido Balde
so far this summer.
And Celtic travel to face Cliftonville in Wednesday's Champions League second qualifying round first leg.
"I haven't spoken to [chief executive] 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," added Lennon.