The Times understands the decision for Luiz - who sat in the stands behind the Chelsea bench for the duration of the 90 minutes - stems from a conversation with Conte on Thursday (2 November) in which he was perceived to have questioned the Italian's authority.
David Luiz had been a crucial player for the Italian boss since returning to Stamford Bridge from Paris Saint-Germain in the final days of the 2016 summer transfer window. And this is according to the former Manchester United captain and now a Sky Sports pundit, Gary Neville.
"At the end of the match I went to shake the hand [of] the referee and the linesman and then my players", he added. I saw him play against Roma [a 3-0 Champions League defeat] the other night and he didn't look anywhere near it.
Chelsea ran out 1-0 winners and Christensen further enhanced his credentials. I have to put my face in every situation, and sometimes make important decisions. Tonight, the best decision for the team was to play with Christensen and to have Ampadu on the bench, young players, really good players for the present and the future.
That is part of the renewal Conte seems to be rolling-out in his side right now: he told us again after United about the plummeting average age of the team.
"I decided this way tonight, maybe I can decide a different way in the future or I can continue the same way". This mindset has yielded spectacular results when selling players - most notably Ramires, David Luiz and Oscar - but led to more problems when buying.
"Sometimes I can make mistakes".
Conte had been prompted to ditch Luiz following the midweek Champions League loss at Roma.
Various reports have since emerged detailing Luiz how reacted to being dropped, with the Times saying he feels like he's been unfairly singled out. "I was very happy for this", Conte said in the press conference following the win over United.