The 6'4" midfielder has already weighed in with five goals in 11 games this season, helping the Blues climb to fourth in the Premier League.
Fellaini has established himself as one of the more dominant midfielders in the league since his £15m arrival from Standard Liege in 2008, and Mirallas believes that there is still more to come.
"We know he's a really vital player for us and let's hope this excellent form continues," Mirallas told Evertontv. "When it comes to the big vital games he's always there and we can rely on him to come up with a big performance.
"You only had to look at his second goal at Fulham on Saturday - he basically made it and scored it all on his own. It was a fantastic effort.
"It's four years now that he's been here and I think in that time you can see how much he's come on as a player, he's really made good progress. He's about to turn 25, so I think he's still got plenty of chance to improve even more."
Mirallas joined the Toffees from Olympiakos for £6m in the summer transfer window.