Chelsea defender Wesley Fofana has undergone anterior cruciate ligament surgery which is set to rule him out of action for several months.
The 22-year-old did not travel with the Blues squad on a pre-season tour to the United States in order to have "specialist medical assessments".
The London club said the tests on Fofana's knee "unfortunately confirmed that an operation would be required".
The France centre-back joined Chelsea for £70m last summer.
"Wesley will now begin his recovery and work with the club's medical department at Cobham during his rehabilitation phase," added a Chelsea statement.