Gary Neville believes the pressure is now on Ruben Amorim as Manchester United head coach and another defeat to Chelsea next week would create "big questions".
United suffered a 3-0 Manchester derby loss to rivals City to sit firmly in the bottom half of the table, with little evidence to show that the club can improve on last season's 15th-place finish.
Their four points from the opening four games means United have made their worst start to a Premier League season in 33 years.
Amorim's side have also exited the Carabao Cup after a humiliating second-round loss to League Two Grimsby Town on penalties, which means United are set for their shortest ever season in terms of games played.
Neville believes a loss to Chelsea next Saturday, live on Sky Sports, could be pivotal for Amorim's future.
"I think there will be some pressure applied to the manager and his rigidity of sticking with the system," said Neville about the ramifications of the loss to City.
"City have beaten them well and comfortably. United had some good moments in the game - the first 15 minutes and the first five minutes of the second half. City, in big moments, have had better players. Foden, Doku, Haaland have stood out.
"There have been times when I have seen United lose this type of match when I feel angry and frustrated.
"I just feel nothing, which is even worse. This has been a nothing of a performance. United have just been beaten.
"With Chelsea next week, another defeat and big questions would start to be asked."
SIRALLEN
31
My opinion: Amorim needs to understand that his 3-4-3 doesn't work in England, especially with no transfers to midfield. Something has to change: tactics or coach. What we need to choose?
baeabmostz
19
likewise me Gary