Manchester United eased the pressure on beleaguered boss Ruben Amorim with a chaotic 2-1 win against Chelsea as both teams finished with 10 men at Old Trafford.

Fighting to avoid the sack after a woeful start to the season, Amorim received a welcome boost when Chelsea goalkeeper Robert Sanchez was sent off after just five minutes of Saturday's clash.

Bruno Fernandes quickly put United ahead with his 100th goal for the club in all competitions in his 200th Premier League appearance.

Casemiro doubled United's lead before the Brazil midfielder was dismissed for a second booking in first-half stoppage time.

Trevoh Chalobah's late reply for Chelsea set up a tense finale, but United held on for just their second win in six matches in all competitions this term.

Having kicked off languishing just above the relegation zone, Amorim's side climbed to ninth place to give the Portuguese coach a little breathing space.

"I felt the urgency. Of course the Sanchez sending off helped us but we had the red card with Casemiro. We like to complicate our game always," Amorim said.

"It is really important to win because I understand the situation around this club.

"I have nothing to say to the critics and most of the time they are right. Today we won and it is a good day for us."

Amorim, who has won just nine of his 32 Premier League matches, shouldn't rest on his laurels just yet.

After presiding over United's lowest top-flight finish since 1973-74 and a limp Europa League final loss to Tottenham last term, Amorim spent big in the summer to overhaul his moribund team.

But United suffered an embarrassing League Cup exit at fourth-tier Grimsby in August before last weekend's 3-0 loss at Manchester City underlined the vast gulf to their rivals.

Former Sporting Lisbon boss Amorim, who replaced the sacked Erik ten Hag in November, insists he retains the backing of co-owner Jim Ratcliffe after speaking to the British billionaire at the club's training ground on Thursday.

Ratcliffe was in the directors box at rain-lashed Old Trafford to see if his support for Amorim was well placed.

- Amorim relief -

With storm clouds gathering for Amorim, he stuck with his much-criticised 3-4-3 formation after joking this week that even the Pope could not make him change his tactics.

The move paid dividends as the erratic Sanchez rashly raced out of his penalty area in a bid to clear before Bryan Mbeumo could reach Benjamin Sesko's flick, but instead sent the United forward crashing to the turf with a reckless challenge.

It was a clear red card for denying a goal-scoring opportunity and there were no complaints from Chelsea about their quickest ever Premier League dismissal.

Filip Jorgensen replaced Sanchez in goal as Maresca took off wingers Estevao Willian and Pedro Neto in a defensive move that immediately backfired.

United went for the jugular and took the lead in the 14th minute when Fernandes, played just onside by Chalobah, timed his run perfectly to meet Patrick Dorgu's header with a clinical finish from four yards.

Chelsea's misery mounted when England forward Cole Palmer was forced off with an apparent recurrence of a groin injury, making it a remarkable three substitutions in only 21 minutes for the Blues.

For once, the luck was all going Amorim's way and Chelsea imploded to gift United their second goal in the 37th minute.

Jorgensen stood rooted to the spot as Reece James's sliced clearance was headed on by Harry Maguire to Casemiro, who nodded home from close range for his first league goal in 11 months.

Casemiro, booked for an earlier foul on Enzo Fernandez, saw red on the stroke of half-time after needlessly pulling back Andrey Santos.

United struggled to find any rhythm in the second half and Chalobah reduced the deficit in the 80th minute when he rose unmarked to meet James's cross with a powerful header that flashed past Altay Bayindir from 12 yards.

It was too late to stop Chelsea slumping to their first league defeat this season as Amorim breathed a sigh of relief.