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A frantic first half got off to the worst possible start for Chelsea when goalkeeper Robert Sanchez was sent off in the fifth minute for a flying lunge at Bryan Mbeumo, up-ending the United forward when he was through on goal.

The hosts, with just one victory to their name previously this campaign, made their numerical advantage count, with Fernandes and Casemiro putting United 2-0 in front inside 37 minutes.

Casemiro's needless dismissal just before half-time evened up the numbers and gave Chelsea a way back into the contest but Trevoh Chalobah's late header was all the visitors could muster as a nervy United held on for a vital three points.

"We start the game really well, in terms of aggression," United boss Ruben Amorim said. "Of course the sending-off helped us but we were in control anyway.

"The second half we managed well but the last 15 minutes were tough for us. We showed that when everything is going well, we still managed to make things difficult. Since the beginning of the game we wanted to win really, really bad."

The pressure has been mounting on Amorim after defeat in the Manchester derby last weekend left the Portuguese with a record of just eight wins from 31 league games since taking charge last November.

The visit of Chelsea offered Amorim the opportunity to start rectifying that meagre return, with the Blues winless at Old Trafford since 2013. Sanchez's rash tackle aided the beleaguered United manager's cause a great deal.

"The game changed after the red card," Chelsea boss Enzo Maresca said. "The best solution would have been to let the player through on goal. All the plan and everything, it doesn't exist anymore.

"Robert is aware of that. I would prefer to be 1-0 down after five minutes than one man down."

Along with the added setback of Cole Palmer being withdrawn through injury, Maresca was forced into an early reshuffle, making three substitutions in the first 21 minutes, the earliest a side has ever done that in a Premier League game.

It was not long before Fernandes added to Old Trafford's euphoria in the driving Manchester rain, the skipper nudging home his 100th goal for United in all competitions.

Casemiro then seemingly put United in an unassailable position after determined work from Luke Shaw to keep alive the attack, the veteran Brazilian heading his first goal of the season.

A second booking soon turned Casemiro from hero to villain as he became the first player to score and be sent off in the first half of a Premier League match since Emmanuel Adebayor for Spurs against Arsenal in November 2012.

Even when chasing the game, Chelsea offered very little attacking threat until Chalobah's header 10 minutes from time. United looked ragged from there on in, but Altay Bayindir remained largely untroubled in the home goal as the hosts held on.

Chelsea is now winless in its past 13 Premier League trips to Old Trafford, its longest such run against any team.