In-demand Sebastian Andersson has insisted he will only leave Union Berlin if he's guaranteed a starting spot at a new club.
The 28-year-old striker was watched by Celtic last weekend in the Bundesliga clash with Paderborn.
Ajax and Schalke are also reportedly interested in the Swedish international who has scored 10 goals in 21 games for club and country this season.
The former Kaiserslautern forward is out of contract at the end of the season, with clubs weighing up whether to sign him on a pre-contract.
And Andersson has admitted he would consider quitting the German capital club if he was given assurances about whether he would be a first team starter.
He said: “For me, the most important thing is to play. Of course, you always have to talk about money - after all there is life after your career.
"But if you don't play, it's no fun.
"I enjoy it at Union. We are not a one-man team. We have no stars."
And Andersson's captain, Christopher Trimmel, admits he's not surprised at clubs including Celtic looking at him.
He said: "Sebastian is a key player for us. He runs after balls very few strikers would chase.
"I have already told our manager to make sure Sebastian signs a new contract. He is absolutely vital to us."
Neil Lennon is on the lookout for a new striker with Leigh Griffiths and Vakoun Issouf Bayo having fitness issues.
It means that Odsonne Edouard is relied upon every week and the Frenchman has also struggled with injury.