The Spaniard says Fellaini, who replaced him as hero of the Blues fans and scored in the midweek clash, is so good that at times he becomes unplayable for opposition defences.
Arteta said: "Fellaini is one of the most difficult midfielders to play against because of his size.
"You don't normally come up against players like that in that position. When he's used in that forward position around the box it is much more difficult to play against him.
"He creates a lot of problems, uses his size well and is a threat around the goal - as you can see from his goals.
"For me, he is much more efficient playing there than when he is deeper.
"I have seen a tremendous improvement in Fellaini, especially in that forward position."