Mainoo scored the winner on the day.
"Mainoo, Kobbie Mainoo, for such a young lad, take his goal out of it, the composure he has shown on the ball for such a young lad has been excellent," Rooney said.
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"Incredible. It's so good to see a player from the Academy coming through and dictating the games for Manchester United.
"He's had some moments when he's struggled as well since he's been in the first team, but I think this afternoon he's been absolutely (brilliant). You look as his composure on the ball, little passes, little one-twos, playing people in behind - I think he's been incredible."