The Belgium international is sidelined with a hamstring problem picked up in October after initially returning to action too early.
Moyes believes the 25-year-old's absence is taking its toll on his side as they push for a top-four Premier League finish this season.
He told the Daily Mirror: "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."