The 27-year-old was replaced after half an hour at the Amex Stadium after slipping on the rain-soaked pitch before leaving the ground on crutches.
Scans have now confirmed that he has picked up a grade 2 medial ligament injury, and though Everton have given no definitive time frame for his recovery, he is likely to be out for some time.
A statement on the Blues’ official website added: “Everton medical staff will speak with the Club’s knee consultant in the coming days to ensure the fastest possible return to play.”
Bernard has been one of Everton’s brighter sparks in a miserable start to the season, and his injury is another blow to under-pressure manager Marco Silva.
He joins a growing injury list at Goodison Park which also includes Yerry Mina, Jean-Philippe Gbamin and Cenk Tosun.