City were awarded a free kick on the left and David Silva's ball in looked to have posed no danger whatsoever.
Lukaku was well positioned to deal with the ball at the far post but when the time came to clear the Belgian could only blast the ball at team-mate Chris Smalling and into the path of Nicolas Otamendi, who made no mistake from six yards.
It wasn't one of City's best goals, but it could prove key come the end of the season.
Check out the goal for yourself below:
Check out this awesome video: Nicolas Otamendi Goal HD - Manchester United1-2Manchester City 10.12.2017 https://t.co/1BoEURyEEF
— Soccer Is Life (@FootballLive30) 10 December 2017