Mirallas has been sidelined for more than two months with a hamstring injury, after he returned too swiftly when the problem first struck in October.
Moyes said: "We've missed Mirallas giving us another edge. It might be that we've not really flowed as I would have liked attacking wise.
"It's shown in one or two results. We've missed him too long. It's a problem.
"He's probably missed the bulk of it, 10 or 12 weeks. He was a big part of what we achieved in that early part of the season when we were at our best and played some good stuff."