Fellaini produced another impressive display on Sunday when he starred in United’s 4-2 derby win over Manchester City at Old Trafford.
The Belgium international did not start a game for United until the end of October, but he has really hit top form since breaking into Louis van Gaal’s side this season.
Van Gaal has described Fellaini as “unplayable” when the former Everton star is on his game, and Rooney believes the Belgian provides a vital outlet for United.
“We like to play out from the back and if teams are pressing us then we have him as an option,” Rooney said.
“He is probably the best in world football at bringing the ball down and getting us out of that pressure and further up the pitch.”
Fellaini’s rise in stock at United is testament to the 27-year-old’s character, after he became a target for the Old Trafford boo-boys when settling into the club last season.
And Rooney thinks Fellaini should take credit for the way he dealt with the negativity that was directed his way.
“Last year was awful for him,” Rooney said. “But in fairness he kept his head down worked hard and come through that tough period.
“It was tough for him but he had the whole team’s support. We knew he was working hard and it wasn’t just him struggling last season, a lot of us were.
“He was the one who took most of the blame for it. It was hard for him. Thankfully the manager has shown faith in him.
“He is showing now what he did at Everton and why Manchester United bought him. He is a quality player. He is a different option for us and it is starting to show on the pitch.”
Rooney reckons United’s victory in the derby was down to exploiting City’s weaknesses in defence.
He said: “We know some City players are not the best at tracking back and defending and tracking runners and we felt we could make them pay for that and we did do.
“We have got players who can score goals from midfield and to be honest we felt that was going to win us the game and it was proved to be.”