Everton winger Bernard faces a lengthy spell out after damaging his medial knee ligament.
The Brazilian suffered the problem after 30 minutes of the Toffees' 3-2 defeat at Brighton on Saturday and was replaced by Gylfi Sigurdsson.
The club confirmed a grade two tear of his right MCL on Sunday, with further assessment needed to determine the quickest route back to fitness.
The club said: "Bernard suffered an injury to his right knee during our game against Brighton.
"Scans have subsequently revealed the Brazilian has sustained a grade two medial ligament injury.
"The club's medical staff will speak with our knee consultant in the coming days to ensure the fastest possible return to play."
Bernard, 27, has made nine appearances for Marco Silva's struggling side this season, and scored in the recent 2-0 win at home to West Ham.
He joined the club from Shakhtar Donetsk for £21.5m in the summer of 2018 and has since scored three times in more than 40 Premier League appearances.