Schalke forward Rabbi Matondo has apologised after a photo posted online showed him wearing a Borussia Dortmund shirt, emblazoned with Jadon Sancho's name and number, while taking part in a gym workout.
Dortmund and Schalke are arch-rivals in Germany's Bundesliga, and fans of the Gelsenkirchen club were unhappy to see one of their players wearing the kit of one of Borussia's star players.
Matondo apologised on Instagram, with the 19-year-old Wales international stating that he had not wanted to cause anyone "grief or trouble".
He wrote: "I am very sad and disappointed in myself that I have let down anyone who feels connected to Schalke. I was naive and put the jersey on for a private training session. The jersey was from my close friend Jadon Sancho. I only wore it because of Jadon, it had nothing to do with Dortmund.
"I did not think about the consequences and that it would annoy many people. There is no excuse for this immature behaviour. I can promise every Schalke player that I have identified with Schalke 100 percent since day one."