Nathan Ngoumou suffered a rupture of his right Achilles tendon following Borussia Mönchengladbach's 1-1 draw against St Pauli on Sunday, the club said.
The injury occurred during a supplementary training session for substitutes and reserve players after the match.
Ngoumou, 25, was brought on in the 86th minute to replace Tim Kleindienst, shortly after St Pauli had equalized to make it 1-1.
According to the Bundesliga club, the striker is set to undergo surgery on Tuesday with a specialist in France.
He will not be available for the remainder of the season and it remains unclear when he will return to the pitch.
"This is devastating news for us and, of course, especially for Nathan personally," sporting director Roland Virkus said in a statement on Monday, promising Ngoumou would receive the greatest possible support.
The Frenchman, who joined Toulouse in 2022, has a contract with the club until 2027.
