Marc Cucurella proved the matchwinner, with Maresca delighted to strengthen their Champions League qualifying hopes.
He said, "Very important. At this stage of the season it's important to win games. We also pay attention in the way we win the game. I think tonight we struggled on the ball because we didn't expect them to be so aggressive man-to-man.
"I used to watch five, six, seven games of the other team. I've never seen them so aggressive man-to-man. So they surprised us a little by that.
"Very, very nice (goal). The assist from the right full-back, the goal from the left full-back is the way I like to play. It's nice. I think Cucu already scored this season an exactly similar goal against Brentford, against Wolves.
"He's in the right position at the right moment, so it's not about how big you are, tall or small. Just to be in the right position at the right moment. Cucu is an intelligent player and he's helping us also with the goals."
Defeating Man Utd very important
Maresca insists the win should not be discounted despite United's struggles.
He also said, "It's very important for sure, but the reason why I was so happy at the end is because United beat City away, drew with City at home, drew with Liverpool, drew with Arsenal.
"So they know how to play against a big team. So I expected, drew with us away, so I expected a very tough game.
"The reason why I was happy was especially because we beat a team that against Arsenal, Liverpool, City, it was difficult to beat them."