The Belgian, who dominated the United defense throughout the first half was also involved in City's second goal, firing a shot which ricocheted off the post straight to the feet of Kelechi Iheanacho who scored what will likely be his easiest goal of the season.
"It's a dream come true to play in a Manchester derby, and to get the winning goal, so I am very, very happy," said the 19-year-old Nigerian striker, who appears to have solidified a spot in Guardiola's starting 11. "I didn't anticipate the shot, actually, but I'm happy it came off me."
Minutes later, a mild Rooney free kick which floated outside of the six-yard box drew City's Chilean keeper Claudio Bravo into making a calamitous mistake by coming off his line. Although Bravo grazed the ball, he couldn't control it, and it bounced straight to Zlatan Ibrahimovic who casually thumped in his fourth goal of the season.
United's second half substitution of Marcus Rashford for Jesse Lingard had an immediate impact, with the 18-year-old using his fresh legs to challenge City's defenders with a series of near-misses, one which bounced into the goal off Ibrahimovic, who was loitering offside.