Wales forward Rabbi Matondo issued an apology on Wednesday to supporters of his Bundesliga club Schalke over a social media post of him training in a Cardiff gym wearing the shirt of arch rivals Borussia Dortmund.
"I want to be honest: I was naive and put on the shirt for a private training session. It was from my close friend Jadon Sancho. I only wore it because of Jadon, it had nothing to do with Dortmund," the Welshman wrote on Instagram.
The 19-year-old said he pulled on the yellow shirt bearing the name of England winger Sancho without thinking "about the consequences and that it would upset many people.
"There is no excuse for this immature behaviour."
The post of Matondo in a Dortmund shirt caused uproar among Schalke fans, many of whom called for him to leave the Royal Blues.
"I would like to apologise to everyone who was annoyed or insulted by my behaviour," the 19-year-old added.
"I can promise every supporter that from day one, I have identified 100 percent with Schalke.
"I will always give everything when I run out for Schalke to prove how much I love the club."
On Tuesday, Schalke sports director Jochen Schneider had said he made it "very clear" to the Wales international what he thought of his "ill-considered action".
Gelsenkirchen-based Schalke are just 30 kilometres (19 miles) from Dortmund, and there is no love lost between the clubs in the Ruhr Valley, the industrial heartland of western Germany.
Matondo managed two goals in 20 matches last season as Schalke stumbled to 12th in the Bundesliga table after having been third in December.