Gylfi Sigurdsson is expected to face up to eight weeks on the sidelines.
The Everton midfielder met with a specialist on Monday in order to assess the extent of the knee injury he sustained during last weekend’s win over Brighton.
Sigurdsson received treatment for the ligament damage before continuing to play, which is thought to have aggravated the problem further.
He is expected to miss the remainder of the Blues’ season and now faces a race against time to feature in Iceland’s World Cup campaign in the summer.
Sam Allardyce said: “We’ll continue to assess Gylfi’s progress on a week-to-week basis and the medical team will work very closely with him, as they would with any injured player, to get him back playing as quickly as possible.”