Erik ten Hag started the match day under pressure but his side dominated from the first minute to the last, securing a 2-1 victory at Old Trafford.
McTominay scored after an energetic first-half display, drilling a low finish across goal. Sanchez had saved a Bruno Fernandes penalty and a host of chances to allow Cole Palmer to equalise against the run of play.
Chelsea couldn't regroup at half-time and McTominay won the match with a far post header in the second half.
Nizaar Kinsella was at Old Trafford witnessing the action...
Robert Sanchez - 8
He made a wonderful save on Bruno Fernandes' early penalty and several point-blank saves to save Chelsea in a poor first half. His kicking could have been better, though, which is why he didn't get a perfect rating.
Marc Cucurella - 4
He made a nervy early clearance early on and struggled at right back before coming off at half-time.
Axel Disasi - 6
Hojlund ran off him too easily early on, but he steadied himself afterwards.
Thiago Silva - 7
He made a heroic block to stop a point-blank Antony shot. Led by example.
Levi Colwill - 6
He captained Chelsea for the very first time at Old Trafford but often found himself exposed at left back on the break.
Enzo Fernandez - 5
Fresh after his first two Premier League goals, he was the best of a bad bunch in midfield, until he produced two air shots on late chances.
Moises Caicedo - 5
He was not at his best level, but he had to guide two more attacking midfielders through a tricky game.
Cole Palmer - 6
Palmer looked poor and uncomfortable playing in central midfield, not his favoured role, but still scored a wonderful equaliser against the run of play.
Raheem Sterling - 5
His dribbling offered Chelsea an outlet, but he wasn't his usual impactful self.
Nicolas Jackson - 4
He slipped to give away possession for United's first goal and often didn't make the right decision in attacking situations. He missed a one-on-one and a close-range header.
Mykhailo Mudryk - 5
Mudryk wasted many big chances as United looked vulnerable to his pace, but lacked end product.
Reece James - 7
Immediately, he set up a huge chance for Jackson and got the better of his rival Garnacho for the most part, making a heroic 74th minute last-ditch tackle.
Armando Broja - 6
Hit the post late on after a good James cross.