Martinez insists that losing the 25-year-old has had minimal impact.
He said: "I've been very impressed with the reaction to Fellaini leaving. When you lose a senior player there is a void that you need to fill and the other senior players in the dressing room have taken responsibility and filled that gap.
"You've got two options; one is to feel sorry for yourselves or two is to see it as an opportunity and to grow with it. We've been doing that.
"We have strong individuals and a team that is now open for competition because there are places to be earned. We can play in different ways and we're flexible. That becomes a real plus."
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