Ousmane Dembele missed Borussia Dortmund training on Thursday, amid speculation that he is wanted by Barcelona.
Dembele, 20, is one of several options Barcelona are targeting following the sale of Neymar to Paris Saint-Germain, with the clubs reported to be negotiating a transfer for the France international.
After joining his Dortmund teammates for the official team photo on Wednesday, Dembele went missing on Thursday.
Ahead of Dortmund's DFB Pokal first round match against Rielasingen on Saturday, Bosz told reporters at a news conference: "It's true, he wasn't there today and to be honest I don't know why.
"We tried to reach him, without any success. I hope that nothing bad has happened.
"I talked to Dembele yesterday. He told me he was in Paris with his friends. He did not say anything about a transfer."
It has been reported that Barcelona could spend up to €150 million to sign the France international, who joined Dortmund from Rennes for around €15m last summer.
Last season, Dembele was directly involved in 31 Dortmund goals in all competitions.
ESPN FC's Borussia Dortmund blogger Stefan Buczko contributed to this report.