Yevhen Konoplyanka described Schalke coach Markus Weinzierl as a "coward" and insisted the club will be relegated from the Bundesliga under his leadership.
Konoplyanka spent the 2016-17 season on loan at Schalke from LaLiga side Sevilla and is expected to remain at the club permanently.
But the Ukraine international's future is in doubt after he revealed a row with Weinzierl, who started him in just five Bundesliga matches as the 27-year-old fell out of favour in the second half of the season.
"He is a coward," Konoplyanka told Ukraine's Football Hub. "He will not stay as the coach of this team, otherwise Schalke will drop to the second tier.
"We talked through a translator. He told me I should look for a new club and I told him: 'Listen, you believe you'll be here longer than me?'
"If I could have said more, I would have asked him more questions. Why is it not possible to talk in advance and explain everything, that's cowardly.
"If you want to get rid of a man who has been bought for 15 million euros, we need more explanation and we have to understand and agree, but he only spoke to me for a minute.
"I will go anywhere just to play. I never would have left Sevilla if I knew. If this coach stays, I am 100 per cent going."