Nagelsmann, whose sacking in March 2023 led to Tuchel's arrival, was tipped to return to Bayern, but has now extended his contract with Germany until 2026
The French winger was substituted after 29 minutes of their 2-1 win at Getafe on Saturday in La Liga after pulling up injured.
“The player will have surgical treatment by Dr Sakari Orava next week in Finland. He will be out for between three-and-a-half and four months,” said Barcelona in a statement.
Dembele is set to miss several big games including La Liga clashes with title rivals Atletico Madrid and Real Madrid, as well as the rest of Barcelona’s Champions League group stage games.
Barcelona signed Dembele this summer from Borussia Dortmund for a record 147 million euros (129.17 million pounds) including add-ons, to replace Neymar, who left for Paris St Germain.