File photo: Chelsea s former head coach Thomas Tuchel. AP
Bayern had signed Nagelsmann for five years just two years ago but nerves have been running high as the perennial Bundesliga champions are struggling to hold onto their lead in the league table.
While Bayern have yet to officially announce the shock decision, German media have widely reported on the switch.
Tuchel -- who Bayern had eyed previously but lost to other top-flight European clubs -- has been in between jobs after he was fired by Chelsea in September.
He is due to lead his first training session of the team at Saebener Strasse as soon as Monday morning, Germany's best-selling daily Bild reported.
And his first fixture will come as soon as April 1, when Bayern meet Borussia Dortmund, a club that Tuchel had led from 2015-2017 before he left for Paris Saint-Germain.
Although Bayern saw off PSG in a shock Champions League catastrophe for the French top-flight club brimming with stars Kylian Mbappe, Neymar and Lionel Messi, the Germans suffered a blow on Sunday against Bundesliga minnows Bayer Leverkusen.
Bayern's sporting director Hasan Salihamidzic gave a withering assessment of his team's form after the poor showing against Leverkusen.
"So little drive, mentality, tackling, assertiveness. I've rarely seen that," he said.
- 'Long-term project' -
The Munich giants are currently struggling with their worst season in 11 years, with just 52 points from 25 played matches, and in second place, a point behind Dortmund.
With just nine games left in the season, Nagelsmann himself had said that the Dortmund match will be a "must-win game", failing which the club risks losing the title.
But beyond nervousness about the title, Bild daily said the club fears that they may miss out on Tuchel by a beat again, said Bild, noting that Real Madrid may be seeking a new coach in the new season and that Tottenham, too, had expressed interest.
That, say German media, may be the real reason for the surprise timing of the decision.
Afterall, as recently as on Monday, the president of Bayern's supervisory board, Herbert Hainer, had sung Nagelsmann's praises, calling him a "super coach who demonstrated in Paris his excellence in tactics and strategies at the highest European level".
Salihamidzic had also recently spoken of a "long-term project" with the trainer in the hunt for titles.
The bombshell to drop Nagelsmann also took players by surprise.
"I did not know," said defender Joao Cancelo on Thursday after playing for Portugal.
But Nagelsmann's relationship with the team had soured over the months, particularly between him and captain Manuel Neuer following the goalkeeper's injury.
After goalkeeper coach Toni Tapalovic, who is close to Neuer, was forced out, the Bavarian captain had voiced his anger openly in several interviews.
"It's like someone ripped out my heart," said Neuer.
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