Chelsea midfielder Michael Essien will be out for between three and four weeks after tearing his right hamstring against Apoel Nicosia.
Essien, 27, limped off during the first half of Tuesday’s 2-2 Champions League draw at Stamford Bridge after scoring a 25-yard equaliser.
A scan on Wednesday revealed that the injury was worse than initially feared.
And the news will call into question his participation at the African Cup of Nations which runs from 10-31 January.
Essien missed most of last season after rupturing knee ligaments but has been a key member of Ancelotti’s side this term.
After the game on Tuesday, the Chelsea manager admitted his team’s second half display had not been good enough.
But he singled out Gael Kakuta, who was making his first start for the club, for special praise, calling the 18-year-old “the future of Chelsea.”
Ancelotti said: “We were playing well and scored two goals. We played with intensity but then lost our concentration. For this I am not happy.
“We have to play for 90 minutes with intensity and concentration. It’s a psychological thing.
“This was not how we want it. I spoke with my players about it afterwards. We had the possibility to close the game out but didn’t play well.
“Kakuta is the only one who could think he played well. He showed his talent and made a fantastic pass for the second goal.”
