Roberto Martinez fears being without Gerard Deulofeu for the festive period after the winger damaged his hamstring against Fulham.
The 19-year-old had to be stretchered off in the second half and limped out of Goodison Park after the 4-1 win over the Cottagers.
And with a quickfire sequence of fixtures continuing with a trip to Swansea next weekend, Martinez admitted the injury was a blow.
He told evertonfc.com: "Gerard's injury is a negative. He's a sprinter and he's pulled his hamstring.
"We don't know how big the tear is but we expect him to be out for a little bit of time.
"We have to be patient to get him ready for an important final third of the season."
Deulofeu has two goals this season and is set to undergo a scan to ascertain the severity of the soft tissue injury.