Guus Hiddink confirmed Kurt Zouma is set to undergo MRI scans following an injury sustained in Chelsea's 1-1 draw with Manchester United.
Zouma was replaced by Gary Cahill in the 59th minute after his right knee buckled upon landing from an attempt to clear an aerial ball.
The French defender's distress was audible while prone on the pitch before he was taken off on a stretcher.
Hiddink could not yet clarify the extent of the injury, but expects to know more following the completion of further examinations.
"Tomorrow [I can tell you] more. Everyone has seen it on the TV, it's a bad injury, but let's see. It's a hyperextension and that's always bad," the Dutchman told Sky Sports.
"I had the same problem once and it hurts a lot, but we will see tomorrow.
"I'm not a doctor, but let's see tomorrow. Let's calm down and see what the MRIs will show."
Jesse Lingard scored just two minutes after Zouma left the field, but Diego Costa preserved Hiddink's unbeaten run as Chelsea boss with a 91st-minute equaliser.
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