Chelsea have confirmed that midfielder Oriol Romeu is expected to be out for six months after undergoing surgery on Monday.
Romeu had anterior cruciate reconstruction surgery and medial meniscus repair after suffering a knee injury in the first half of Saturday's win at Sunderland.
The 21-year-old returned to London for assessment after the game instead of flying to Japan with the rest of the squad for the Club World Cup.
Romeu lasted just 20 minutes of the 3-1 win at the Stadium of the Light before being forced off and replaced by Oscar.
Chelsea had initially hoped he might have been able to join up with the Blues squad in Japan later this week, but further assessment revealed the need for surgery.
Romeu has made nine appearances for Chelsea in all competitions this season, scoring in the Capital One Cup win over Wolves in September.