When Amorim was appointed last November, he made it clear that his preference was to wait until the end of the season before joining the club.
However, United’s board stressed it was a “now or never” offer.
That, coupled with the perceived mess that Amorim inherited and his desire to play a completely different formation, meant many gave him a free pass last term.
The logic was that, after the lowest Premier League finish (15th) in the club’s history, a full pre-season with new signings would be a fresh start.
But evidence of that has been in short supply.
“What an embarrassment for Manchester United,” former England striker Chris Sutton told Radio 5 Live.
“Where does Ruben Amorim go from here? The perception of him now, I don’t think he gets over this.
“If I was Ruben Amorim tonight there is not a prayer that I would have made eight changes. I would have played my strongest team, try to put the game to bed and then make changes.
“That was an error from him in my book. He will struggle to come back from this. The pressure this season is well and truly on now. If I’m him, I am officially worried.”
Last season’s 15th-placed finish in the Premier League means United are without European football this season for just the second time in 35 years.
But defeat in the Carabao Cup has scuppered one possible route back into Europe.
It increases pressure on performances in the league, where their two opening fixtures have produced just one point – leaving them 16th.
The pressure on Amorim is not what minority owner Sir Jim Ratcliffe envisaged when he sanctioned a move for the Portuguese last November.
But, even when results were poor last season, Ratcliffe told BBC Sport in March that Amorim was “an excellent manager and I think he will be here for a long time”.