Casemiro was sent off for a second yellow card as Manchester United were reduced to 10 men, alongside Chelsea who also saw a player receive a red.
On a chaotic afternoon at a drenched Old Trafford, a frenetic half ended with both teams seeing a player sent for early baths.
Chelsea goalkeeper Rob Sanchez saw red after five minutes when he chopped down Bryan Mbeumo, who was through on goal.
United raced into a two-goal lead with Bruno Fernandes and Casemiro both getting on the scoresheet.
Casemiro was the second man to be sent off in a chaotic first-half between Manchester United and Chelsea
|REUTERS
But as the half drew to a close, Casemiro's afternoon was cut short when he saw a second yellow for hauling down Andrey Santos.
Referee Peter Bankes had no hesitation, having booked the Brazilian earlier in the contest for sliding in on Enzo Fernandez.
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Speaking on BBC Radio 5Live, former Chelsea star Pat Nevin could not believe his eyes.
The Scotsman said: "The first yellow card was a stupid one.
"Casemiro getting booked for that which is basically not being quick enough and dragging someone back, every Manchester United fan would have thought that's exactly how he would have got sent off."
Casemiro had been penalised for hauling Andrey Santos down
|REUTERS
United were in need of a lively display with Ruben Amorim under immense pressure prior to kick-off, especially amid reports of a meeting with club co-owner Sir Jim Ratcliffe this week.
And they gave him exactly that and put Chelsea under immediate pressure, who themselves are reeling from a defeat to Bayern Munich midweek in the Champions League.
A long ball from goalkeeper Altay Bayindir was flicked on by Benjamin Sesko, with Mbeumo racing on.
Sanchez came flying out of his goal and scythed the former Brentford star down, leaving the referee no choice but to send off the Spaniard.
United only ramped up their pressure further as Enzo Maresca made three changes before 20 minutes with Cole Palmer, Pedro Neto and Estevao Willian all coming off for Tosin Adarabioyo, Filip Jorgensen and Santos.
And the home side were rewarded for their endeavour when Fernandes opened the scoring, breaking Chelsea's poor offside trap to net his 100th United goal - despite a lengthy VAR check
Chelsea's shocking defending only continued when a corner was awfully cleared by Reece James after Jorgensen failed to come and collect the cross.
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The England defender hooked the ball high up into the air as the entire Blues defence watched the ball in the sky.
That left Casemiro completely unmarked as Harry Maguire won the header, knocked it down to the Brazilian from five yards out, who hammered it home to double United's lead.
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