Rooney always had faith in Fellaini's ability
Fellaini continued his resurgence with another impressive performance against rivals Manchester City on Sunday, scoring a goal as United claimed bragging rights via a 4-2 derby victory.
The Belgium international has become a mainstay under Louis van Gaal in recent months, a far cry from Fellaini's first season at Old Trafford in 2013-14, when he was made a scapegoat by fans for the club's poor campaign following his £27.5 million move from Everton.
Fellaini was deployed as a holding midfielder by former boss David Moyes but the 27-year-old has been pushed further up the field, which has kept star loanee Radamel Falcao warming the bench.
"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."
Rooney added: "Last year was awful for him. 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."