At the end of the German League game, in which Bayern Munich lost 3-2, Nagelsmann strongly criticized referee Tobias Welz, namely in relation to the sending off of central Dayot Upamecano, in the ninth minute, although the coach later asked sorry.
"Emotions are part of sport, but I have to apologize for the choice of words for referee Tobias Welz," the German champions coach later wrote on Twitter.
Bayern Munich, who on Saturday suffered their second defeat in the Bundesliga, lead the competition on equal terms with Union Berlin and Borussia Dortmund.
On Sunday, the Bavarian team receives precisely Union Berlin, in a game of the 22nd round of the championship.