But the 24-year-old has drawn a blank in each of United's past five games and Mourinho interpreted boos from some of the Old Trafford faithful when he replaced Marcus Rashford with match-winner Anthony Martial during Saturday's 1-0 win over Tottenham as a lack of support for Lukaku.
Anthony Martial replaced Marcus Rashford in the second half and the France worldwide scored an 81st-minute victor with a scruffy finish at Old Trafford.
But Mourinho said: "The fans pay for their tickets and are free to express themselves the way they want". It's not one goal or the ball goes against the post or a save by the opposition keeper that can make Romelu's contribution below the top level.
United boss Mourinho described the injury as a "long-term" issue towards the end of last month.
"He is playing well and yes I have to protect my players and Romelu always deserve that because what he does for the team is fantastic and to play football for a striker".
"I think Romelu is one of the players who should be untouchable in terms of respect from everyone", the manager told the media on Monday afternoon.
"They pay the ticket, they can do what they want", Mourinho said.
The manager believes the club have made progress since winning the Europa League last season, when they also finished in the relatively low position of sixth in the Premier League.
"I don't think he was quiet [against Spurs] at all, ask [Toby] Alderweireld and [Jan] Vertonghen and [Eric] Dier. they are some of the best centre defenders in football". If United win they are guaranteed qualification for the knockout stage.
The two-time Champions League winning manager also singled out match victor Anthony Martial for individual praise: "Sometimes he starts the match and his contribution is good but not scoring so many times as he does coming from the bench. Nobody here thinks about Chelsea", he added, referring to this weekend's Premier League game at his former club. "We played in the Europa League and won it, so my team are now playing in the Champions League".