The club confirmed the XI and substitutes for the match at London Stadium prior to kick-off:
James Robson of the Evening Standard noted that Sanchez's absence wasn't due to any fitness issues:
The forward was brought in from Arsenal in the 2018 January transfer window, but he has struggled to reproduce his Gunners form in United red.
In five Premier League appearances for the club in 2018-19, he's yet to find the net.
However, his poor run at Old Trafford extends beyond this term, as he only netted twice in 12 outings for the club in the Premier League last season.
He's on a major drought:
For manager Jose Mourinho, it was a big call to make at a time when he is under pressure to see his team perform.
The match at London Stadium felt vital for the Portuguese coach, and the dismal manner of United's performance in the defeat will not have done anything to alleviate the pressure on Mourinho's shoulders.
Per journalist Mootaz Chehade, Sanchez's absence pointed towards a shift in setup from United for this encounter:
Jacob Steinberg of the Guardian is predicting the forward will be on the move soon:
Sanchez has proved himself to be one of the best players in the Premier League in recent years, scoring 60 goals in his three-and-a-half years at the Emirates Stadium in the top flight.
On paper, he should be an ideal player for Mourinho's side, as he presses from the front, doesn't shirk any defensive responsibilities and is typically productive in front of goal.
However, Sanchez doesn't deserve to be in the starting XI after a string of below-par displays. Not having him in the squad at all to potentially make an impression from the bench did feel like something of a statement from the United boss, though.