Chelsea fear Ruben Loftus-Cheek ruptured his Achilles tendon during their friendly in the USA on Thursday morning and faces several months on the sidelines.
Loftus-Cheek collapsed in agony during the second half of their 'Final Whistle on Hate' fixture against the New England Revolution, which they won 3-0.
More scans are due to take place to confirm the nature of the problem, but he is already certain to miss the Europa League Final against Arsenal on May 29 and England's bid to win the Uefa Nations League next month.
The 23-year-old had to be carried off by two members of Chelsea's medical team and was seen leaving the Gillette Stadium, which is the home to NFL champions the New England Patriots, on crutches and wearing a large protective boot on his left foot.
Coach Maurizio Sarri didn't speak afterwards to give an update, but left back Marcos Alonso provided some insight.
He said: “It doesn't look very good to be honest. Fingers crossed, it will not be too much and that he can be running as soon as possible.
“I think it's his achilles. I don't know if he felt (it happened when) he was on his own. We are all very sad about this. It was very, very important not to get any injuries in this kind of game. Hopefully it won't be too bad.”
He is the top scorer out of Chelsea's central midfielders with 10 goals, one of which was in the Europa League semi-final against Eintracht Frankfurt, despite getting less minutes than Jorginho, Mateo Kovacic, N'Golo Kante and Ross Barkley.