Lukaku suffered a head injury in the eighth minute of United's draw against Southampton on Saturday and received oxygen before he was carried off on a stretcher.
"Romelu is not a problem, maybe Romelu can play," Mourinho revealed. "Was precaution by the doctor [to rest him], was following the rules, to protect the situation, but Romelu was in the dressing room, never lost memory, he knew what happened.
"So he didn't come today to follow the protocol. But he wanted to play, we had to stop him to play because he was saying, 'I feel amazing, I I should go and play.' But to follow the protocol and to protect the player, we decided he wouldn't play."