Luiz, despite making a solid start to the 2017/18 season, didn't even make the bench at Stamford Bridge and was forced to watch from the stands.
Twenty-one-year-old Andreas Christensen took the Brazilian's place in Chelsea's starting XI and fortunately for Conte, he had a stormer.
Christensen was up against one of the Premier League's best strikers in Romelu Lukaku but kept him quiet throughout the 90 minutes.
When asked why he dropped Luiz for Christensen, Conte explained it was a tactical decision and that the youngster is in "really good form".
"It was a tactical decision," said the Italian. "There is Christensen in really good form and also we have [Ethan] Ampadu who is a good young player. The club like to play with the young player.
"We are conceding many goals so you have to find stability and solidity. We have to be patient with the team and defend in the right way."
Questions are now being asked of whether Luiz has a future at Chelsea, let alone if he can regain his place in the starting XI.
In light of Luiz being dropped for the United game, a Facebook post from 2013 has re-emerged and it makes for pretty awkward reading.
Four years ago, during Luiz's first stint at Stamford Bridge, he posted a picture with Christensen alongside the caption: "He definitely will have a great future. My friend Andreas Christensen."
LUIZ ON CHRISTENSEN IN 2013
It's now 2017 and Christensen, who Luiz said had a "great future", has replaced the 30-year-old at centre-back for the foreseeable future. How times change.
Luiz now has a point to prove at Chelsea, with Conte also explaining after the United win that while his future is unclear, he can still get back into his starting line-up.
"I don't know," said Conte when asked if Luiz will stay at Chelsea. "He has to work really hard otherwise [he is] on the bench or in the stand. But it can change, it can change."