The Citizens were reduced to 10 men after Phil Foden was sent off in the 23rd minute for violent conduct.
Despite United hitting the woodwork twice, Haaland broke the deadlock on the hour with his ninth goal in eight top-flight appearances against the Red Devils.
“It hasn’t happened a lot in my lifetime to go down to 10 men so it’s kind of a new experience to win with 10 men. It’s amazing to be honest,” Haaland was quoted by Man City website.
“We had to lay low, which is not ideal because we like to keep the ball and like to try and score goals.
“These times you have to go low and take your chances when possible and we managed to do it well.
“This shows personality and character to keep the ball, away at Old Trafford with 10 men. That is personality. We had to change the plan a bit, Enzo (Maresca) gave us a plan and we played a bit differently to our strengths. Counter attack a bit more.”