Two goals from Marcus Rashford and Martial stunned Manchester United in the Sunday English Premier League fixture.
The England forward made it one nil in the 23rd minute converting a penalty.
Six minutes later French forward Martial made it two as he sent the ball low past Ederson.
In the 85th minute Argentine Nicolas Otamendi pulled one back for the host as his header goes past De Gea who was unsure as to whether to go for it or not.
Fred of United was hit by objects as he tried to take a Conner kick as the match was stopped for a few minutes to calm the situation.
The loss condemns Manchester City 14 points a drift of league leaders Liverpool who won their fixture.
Manchester United sit at the 5th position, five points below Chelsea who were beaten by Everton on Saturday.