Ruben Loftus-Cheek will have surgery to repair a ruptured Achilles tendon, Chelsea have confirmed.
The England midfielder suffered the injury during the Blues' 3-0 friendly win over New England Revolution in Boston on Wednesday.
Loftus-Cheek has hit a rich vein of form over recent weeks but will miss the Europa League final against Arsenal in Baku at the end of this month as he begins a long road to recovery.
England manager Gareth Southgate had already left 23-year-old out of his squad for the Nations League Finals earlier on Thursday.
"I went to bed last night and Ruben is in the squad and playing brilliantly and now it is a huge disappointment for him. He misses now a major European final and a brilliant opportunity with us," Southgate told reporters.
"In particular he has really come on in the last few months and looked strong and playing with a lot of confidence and I am really disappointed for him in particular."
Loftus-Cheek scored 10 times across 40 appearances in all competitions for Chelsea this season.
His team-mate for club and country, Callum Hudson-Odoi was struck down by a similar injury during last month's 2-2 Premier League draw against Burnley.