The Serie A club have been heavily linked with a move for the midfielder, who joined United from Everton for £27m last summer but Benitez accepts that the deal is out of his club's hands.
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"Fellaini is a Manchester United player - it does not depend on us but it depends on them," he told the press.
"He is a great player but he is at another team. It appears he is waiting for Van Gaal. We will have to wait.
"Is he the right player? We have players who work very well, while it also depends on who leaves. It is not easy to name names.
"What features in a midfielder am I after? Players who have the mindset to win every game, then we can start to talk about the technical quality. With our same mentality but with a little more quality."
Fellaini made 16 Premier League appearances for United last season but failed to score as he became a source of frustration among many fans at Old Trafford.