Bent was back available after missing three games with an ankle problem but Lambert instead preferred rookie Jordan Bowery as his back-up striker on Saturday evening.
The England player had already lost his starting place to Christian Benteke before the injury and there has been speculation that he could leave Villa during the January transfer window.
When asked why Bowery was preferred to Bent, Lambert said: "That's my decision. I'm the man that picks the team.
"Listen, it's my choice and I've 26, 27 lads that I have to look after. Everybody's in it together and I need everybody in it together.
"I know the disappointment when you don't play. I was dropped about five million times as a player, but that's the nature of the game.
"If you could pick everybody you would. Darren's still part of it but Ive got 25, 26 guys to take care of and think of."
Lambert reiterated that there is no rift with Bent but did not say whether he will be involved in some capacity for Tuesday's home game with Reading.
He added: "Darren is fine. There's nothing at all. The most important thing is Aston Villa Football Club."