It looks like unexpected pressure is building on Bayern Munich boss Julian Nagelsmann with the German Football Federation launching an inquiry into his heated outburst aimed at referee Tobias Welz and his team of match officials in the wake of the Bavarians' 3-2 defeat at Borussia Monchengladbach on Saturday.

Nagelsmann tried talking to the referee immediately after the game, then rushed past journalists in the mixed zone to go to the officials’ dressing room.

"Is he messing me around or what?" Nagelsmann reportedly roared on his way before he knocked on the officials’ door.

Kicker magazine reported Nagelsmann spent about a minute in the changing room, then emerged just as angry when he came out.

"My god, my god, a soft-rinsed pack!" Nagelsmann said.

Nagelsmann later apologized for his "choice of words," but it hasn’t stopped the federation from looking into potential unsporting behavior. It said Sunday it had asked the Bayern coach for a statement and will decide what action to take after it has been evaluated.

Nagelsmann was furious, over central defender Dayot Upamecano’s early red card – a decision that left his team with a player less from the eighth minute.

Welz sent off Upamecano for a light touch on Alassane Plea’s shoulder as the French forward rushed through on goal. There was only minimal contact, but it was still enough to put Plea off.

"The Gladbach player gets himself in front of the Bayern player just in front of the penalty area, gets the contact, and thus loses his balance, although he still tries to keep running," Welz said Sunday. "The striker wants to score the goal. He’s going on his own toward the goalkeeper. Why would he throw himself down?"

Nagelsmann saw it differently.

"It’s simply not a red," he said after the match.

Nagelsmann is also under scrutiny for taking off team captain Thomas Muller to compensate for losing Upamecano rather than the out-of-sorts Serge Gnabry or inexperienced Ryan Gravenberch among others.

Muller has supported injured team captain Manuel Neuer, who is upset with Nagelsmann over the club’s decision to fire his friend and goalkeeping coach Toni Tapalovic on Jan. 23.

Nagelsmann said it was a "crappy decision" to take Muller off, but he had no choice. The 33-year-old Muller is Bayern's most experienced player, having made his 429th Bundesliga appearance for the club. Only goalkeeping great Sepp Maier, with 473, has more.

Nagelsmann has seen his team squander a four-point lead since the Bundesliga resumed after the winter break. Bayern drew its first three league games upon its return.

Saturday’s loss stretched Bayern’s winless run against Gladbach to five games across all competitions, including the 5-0 rout Gladbach inflicted in their German Cup meeting last season. The 35-year-old Nagelsmann has never seen his team beat Gladbach since he became Bayern's coach.

Borussia Dortmund and Union Berlin are now both levels on points with Bayern, which still leads the Bundesliga because of its superior goal difference.

Bayern next hosts Union in Munich on Sunday. In contrast to Bayern, Union is under no pressure going into the game, having already exceeded all expectations and reached its season target of 40 points for league survival.

Unbeaten so far in 2023, Union missed the chance to take the lead on Sunday by drawing with last-place Schalke 0-0.

Union coach Urs Fischer said he could live with the missed opportunity "quite well."

Union, promoted in 2019 and enjoying its best-ever season, doesn’t need to be first. Bayern does. All the pressure is on Nagelsmann.