Everton have confirmed Salomon Rondon sustained a hamstring injury in the defeat to Liverpool.
But manager Rafa Benitez was unable to put a timeframe on his recovery and said they must wait for the striker to undergo tests at Finch Farm.
Rondon had produced his best performance for the club before he was forced off after 58 minutes at Goodison last night.
Benitez said: "Rondon, he was working hard to be fair. He was working really hard, doing well.
"He felt his hamstring but we need to wait and see what the physios are saying."
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The injury to Rondon leaves Benitez with limited striker options for Monday night's crunch game with Arsenal.
Richarlison returned from suspension to play in support of Rondon last night and the Brazilian is expected to lead the line at the start of next week, in the continued absence of Dominic Calvert-Lewin.
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Everton are not expecting the No9 to be fully fit again before Christmas with Benitez saying, earlier this week, the club "need to be careful" with his return to the side.
Cenk Tosun came off the bench against Liverpool for his first appearance of the season, whilst youngster Lewis Dobbin was among the unused substitutes.
Ellis Simms played for Everton's under-23s on Wednesday night and so was not involved with the senior squad.
