Luiz lost his place in the starting line-up to Andreas Christensen for the clash with second-placed United, with the defender not even making the bench as 17-year-old Ethan Ampadu was preferred.
Without Luiz, the Blues went on to take all three points, as Alvaro Morata's header sealed a 1-0 win.
And, when asked about the defender after the game, Conte could not guarantee his continuity at Stamford Bridge.
“Luiz's future? I don’t know. He has to work really hard otherwise he will be on the bench or in the stand," the manager explained to the BBC.
Prior to the game, Conte was adamant there is no issue with Luiz, who started the midweek 3-0 drubbing at the hands of Roma, simply saying Christensen and Ampadu deserve their opportunities.
"It is a tactical decision," the manager told Sky Sports. "There is Andreas Christensen in good form and we have Ethan Ampadu who is a good, young player at the club.
"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."
In his post-match news conference, Conte attempted to clarify the state of affairs by reverting to his pre-game position that Luiz had been dropped for tactical reasons.
He added: "It's only a tactical decision, only a tactical decision, only a tactical decision. It's normal. This can happen to every one of my players, if I see that they are not in good form. It's normal."
Asked if he meant to sound uncertain about David Luiz's future at the club in the previous interview, Conte appeared to go back on his comments.
"No... [checks with press officer]. I think that I have a squad. I have a squad with 16 senior players, and five young players.
"I have these players, and then I repeat I have to make the best decision for the team. I try to do this."