The 18-year-old attacking midfielder was a surprise addition to Nagelsmann's squad after his meteoric rise this season with domestic double winners Bayern Munich, but now his World Cup participation looks to be in doubt.
"Lenny was injured today. We have to see what it is. It does not look good," Nagelsmann told a press conference ahead of Saturday's friendly with the World Cup co-hosts.
"We will go to the hospital to get scans calmly. Don't start driving after him. Leave him alone," Nagelsmann said. "You'll get the news. He needs to process the situation himself.
"We need a diagnosis first and we will inform you whether we can hopefully continue with him or nominate someone else."
The four-time champions have set their sights on winning the title after shock first-round exits at the previous two tournaments, in 2018 and 2022.
Germany have not tasted international success since winning their last World Cup title in Brazil in 2014.
They are in Group E and kick off their campaign against Curacao on June 14th before facing the Ivory Coast and Ecuador.
FIFA World Cup 2026
The 2026 World Cup will be held from June 11th to July 19th in the United States, Canada and Mexico. The tournament will feature 48 national teams and will be played in 16 modern stadiums.
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