Bundesliga club Schalke have taken a dim view of a social media post in which Welsh forward Rabbi Matondo is seen in a Cardiff gym wearing the shirt of bitter rivals Borussia Dortmund.
Matondo, 19, wore a shirt bearing the name of England winger Jadon Sancho, one of Dortmund's star players.
"I made it very clear to Rabbi Matondo on the phone what I think of such an ill-considered action," Schalke sports director Jochen Schneider told AFP subsidiary SID on Tuesday.
"He is only 19 years old, but that still shouldn't have happened.
"We clearly told him that he had to show the right reaction to his misconduct both on and off the pitch."
The post of Matondo was quickly taken down after a flurry of angry comments from Schalke fans, including "There's the door!" and "Rabbi Matondo wearing a BVB jersey in the gym. You couldn't make it up!"
Several Royal Blues followers said they hoped Matondo was wearing the shirt for a bet.
However, one furious supporter wrote: "When it comes to football, it's just a no-go for me and I don't care whether it was a bet or if he likes the colour yellow."
Despite their clubs' deep rivalry, Matondo is friends with Dortmund star Sancho. The pair spent their formative years at Manchester City before moving to rival German clubs.
Matondo joined Schalke in January 2019 for around €10 million (Dh41.6m), while Sancho, 20, signed for Dortmund in August 2017 for a fee of £8 million (Dh37m).
While Sancho netted 17 goals and created as many assists as Dortmund finished Bundesliga runners-up to champions Bayern Munich in 2019/20, struggling Schalke and Matondo had a season to forget.
He managed two goals in 20 matches as his side went winless in their last 17 games to finish 12th in the league table having been third in December.