The injury was confirmed by Bayern's medical staff on Friday following examinations, with the club stating Karl "will be sidelined for the time being." Sky Germany reports he is expected to be out for around three weeks.
The timing could hardly be worse. Karl will miss Saturday's Bundesliga match at St. Pauli and, more significantly, the Champions League quarter-final second leg against Real Madrid at the Allianz Arena on Wednesday. Bayern won the first leg 2-1 at the Bernabéu and are well placed to advance.
He will also miss the DFB-Pokal semi-final against Bayer Leverkusen, and a potential European semi-final looks beyond him too.
Karl has been one of the revelations of the season across Europe. Born in 2008, the attacking midfielder has registered nine goals and seven assists in 36 appearances in all competitions this season, including seven outings in the Champions League.
His impact at the Bernabéu first leg was telling — even from the bench. His absence was felt in a tight match Bayern edged 2-1.
The injury also casts doubt over his World Cup prospects. Germany coach Julian Nagelsmann handed Karl his first two senior caps during the recent international break, praising the teenager warmly.
"He made a great impression — confident, but also humble," Nagelsmann said after the call-up.
Karl's market value is estimated at €60 million by Transfermarkt, and his contract with Bayern runs until 2029. He had previously undergone trials at Real Madrid as a child before ultimately joining the Bavarian club's academy.
Bayern now face their most important week of the season without one of their brightest sparks.