Patterson, 20, has started off the Premier League campaign in superb form for the Toffees. He’s featured in all seven games and kept captain Seamus Coleman out of the team.
But while Clarke could not give a full prognosis of Patterson’s problem, he is set for a period on the sidelines.
Clarke told Premier Sports: “Nathan Patterson, it doesn’t look too good when they go off on a stretcher. He, obviously felt something on his leg.
“We’ll just have to wait and see how it pans out. Hopefully, it’s not too serious, and hopefully he’s not out for too long.”
“Two players light (with Duncan Turnbull also injured). I’ll probably need to call up at least one, maybe two players.”
Everton’s next Premier League fixture is against Southampton at St Mary’s on Saturday 1 October.