The Belgian international was forced off the pitch in the first half of his team's 1-1 draw against Liverpool this afternoon.
"It seems bad," Everton's official website quotes Martinez as saying. "It's a hamstring. We're going to assess him in the next 24 hours.
"Kevin has been in great form and we've got a lot of games coming up. It's very unfortunate for us."
Mirallas has scored three goals for Everton so far this season.