Giuseppe Rossi injured his left knee during Celta Vigo's 2-0 loss to Eibar in the Spanish league on Sunday.
The 30-year-old American-born Italian has suffered three serious injuries to his right knee.
Rossi was substituted in the 35th minute after an awkward fall while disputing the ball. He walked off the pitch and sat on the bench.
The club said he will undergo tests to determine the injury.
''He felt a pop in his left knee on a move. We hope it isn't serious,'' Celta coach Eduardo Berizzo said.
Rossi scored a hat trick for Celta in a 3-1 win over Las Palmas in the league on Monday.
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