Lukaku joined Inter from Manchester United over the summer transfer window and has gone on to make a positive start to life at San Siro.
But a quadricep injury that has been dogging him for over the past week appears finally to have caught up with him, causing him to start as a substitute in Inter's 3-1 victory over Sampdoria on Saturday.
And he will not form part of the Nerazzurri's matchday squad on Wednesday as they look to upset Barca at Camp Nou.
“Lukaku isn’t here, he had a slight muscular issue in the quadricep that he has been carrying for 10 days,” Conte explained to reporters.
“He was continuing to feel the problem and the tests were negative, but we preferred in any case not to risk him. We have to rely on how the player feels in these cases.”
Inter will be hoping for a positive result against the Liga champions to get their Champions League campaign on track, having been held to a draw against Slavia Prague in their first group outing.
Wednesday's clash, however, is not the only big fixture on the horizon for Conte's men.
The new Nerazzurri coach welcomes former club Juventus to Milan at the weekend in a match that already looks likely to have implications in the Scudetto race; and he is hoping to recover Lukaku for that title six-pointer.
“It’s not a tear, so we are fairly relaxed and will focus on that only after tomorrow’s game with Barcelona," he added on the injured forward.