Moyes, who signed Fellaini during his time as Everton manager, had met with disappointment at every turn in the transfer window until he completed a £27.5 million ($42.7 million, 32.4 million euros) deal for the Belgian midfielder shortly before the deadline passed.[break]
The fee was £4 million more than United would have had to pay had they triggered a clause in Fellaini´s contract that expired at the end of July and made him the club´s fourth most expensive acquisition after Dimitar Berbatov, Rio Ferdinand and Wayne Rooney.
However, having spent four years working with Fellaini at Everton, Moyes is confident he will improve his squad.
"Marouane is not someone you want to be playing against," Moyes told United´s in-house television channel, MUTV.
"He has attributes other people don´t have. And with those qualities he has, I am sure he will be a big player for Manchester United."
Following Paul Scholes´s retirement and with Darren Fletcher still recovering from a chronic bowel condition, United had looked short in central midfield, and Moyes says Fellaini will give him a variety of options in the middle of the pitch.
"He can play higher up the field if we need him to, just behind a main striker," said Moyes.
"He can play as a defensive or holding midfield player. In fact he is comfortable in any position in the middle of the field.
"It was definitely a position where people would say we were short, not just of numbers, with the likes of Paul Scholes retiring and Darren Fletcher not being ready to come back just now. I am glad we have got him."
Fellaini is currently on international duty with Belgium, but could make his United debut at home to Crystal Palace when the Premier League resumes on September 14.
United have also announced the signing of 17-year-old full-back Saidy Janko from FC Zurich for an undisclosed fee.
Saidy, who was born in Switzerland to a Gambian father and Swiss-Italian mother, recently underwent a trial spell at United´s Carrington training base.
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