Chelsea centre back Levi Colwill suffered an anterior cruciate ligament (ACL) injury in training, the Premier League club said late on Thursday, with the 22-year-old expected to miss much of the new season.
The England international underwent successful surgery on the injury, Chelsea added.
It typically takes some six to nine months for a player to recover from an ACL injury and return to action.
"The 22-year-old had reported back to Cobham for pre-season at the start of this week, before unfortunately sustaining the injury in training," Chelsea said in a statement.
"Medical assessments confirmed surgery would be the required course of action.
"Levi will now begin his recovery and be supported by the club's medical department at Cobham during his rehabilitation phase."
Colwill made 43 appearances in all competitions for Chelsea last season and helped the West London side win the Club World Cup in July.
Chelsea, who finished fourth last season, open their 2025-26 Premier League campaign against Crystal Palace at Stamford Bridge on 17 August.