Bosz confirmed the 21-year-old has a made a decision over his future.
Reports claim the German playmaker has already agreed personal terms with the Premier League side.
Bosz told Deutschen Presse-Agentur: "I talk to Kai a lot and try to feel what all this does to him.
"For example, these daily rumours about transfers.
"I keep finding that Kai is handling all of this very well.
"I don't think he will stop playing at 29 [like Andre Schurrle].
"Yes, I know what he wants to do, but of course I'm not going to give it away, it's up to him."
When asked about when a decision will be made, Bosz replied: "I couldn't say.
"Perhaps the crucial call will come today, maybe next week, maybe not at all."