Alarmingly, news of this tactical modification emerged on Monday afternoon, sparking anxiety within the club that Bournemouth may have been tipped off about their game plan hours before kick-off.
The Daily Mail claim that United are determined to trace the origin of the breach to ensure similar episodes don't happen again and have launched an investigation.
When quizzed about the alteration following Monday's match, Amorim deflected the question back towards journalists.
He said: "That is for you to discuss, not for me. I know that you guys know that I trained this week with a back four. I don't know how, but that is a good thing for you guys to discuss this week."
This isn't the first breach under Amorim's tenure, with United reportedly 'furious' in September following stories claiming captain Bruno Fernandes was 'unhappy'. The club's starting XI was also disclosed prematurely ahead of the Manchester derby last December.
Several other stories were leaked last season, including those regarding squad issues and team news, with United's hierarchy taking steps to identify the source within the camp.
Previous managerial reigns at M16, under Erik ten Hag, Ole Gunnar Solskjaer, and Jose Mourinho, were similarly dogged by leaks.
United began brightly against Andoni Iraola's side, with Amad deservedly putting Amorim's men a goal ahead in the first half before they were pegged back by Antoine Semenyo's equaliser.
Casemiro nodded United back in front on the stroke of half-time from a corner, before a quick-fire double at the start of the second half gave Bournemouth the lead. Fernandes then equalised with a stunning free-kick before Matheus Cunha put the Red Devils ahead again.
But once more, United could not hang on to their lead with Eli Junior Kroupi pulling the Cherries level once more. The hosts have now climbed to sixth in the top-flight table, with Bournemouth edging themselves up to 13th.
"If you understand a little bit," Amorim said. "If you follow the club like I follow the Premier League for so long, you [know you] have not just the duty to try to win the games. The way you try to win the games is so important for the fans.
"Of course they are desperate to win, but also, I feel that they are desperate to be inspired when they come to Old Trafford. Today was inspiring, I think, also because of Bournemouth. But in the end that's the feeling of frustration of not winning the game."