The Belgian was the sole deadline day arrival at Old Trafford, leaving many Red Devils fans underwhelmed, but the Scot is confident his £27.5 million man will make an impact
The Belgian international completed a £27.5million move from Everton on transfer deadline day, and Moyes, who managed Fellaini for five years at Goodison Park, has hailed his arrival.
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"Marouane is not someone you want to be playing against," he said.
"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."
Everton made public a £28m combined bid for Fellaini and left-back Leighton Baines earlier in the window, which was rejected and described as "derisory".
United went on to pay £500,000 more than that figure for their new midfielder alone, with the pursuit of Baines ending in disappoinment.
The Old Trafford outfit also announced the signing of 17-year-old defender Saidy Janko from FC Zurich on Tuesday.
The Swiss teenager is the club's third signing of the window following Fellaini's big-money arrival and that of young full-back Guillermo Varela from Penarol.