Lukaku, who has netted 11 goals this season since joining United from Everton for £75 million ($NZ144 million), extended his barren streak to five games in Saturday's 1-0 victory over Tottenham Hotspur but did set up Anthony Martial's winner.
"I think Romelu is one of the players who should be untouchable in terms of respect from everyone," Mourinho said ahead of tomorrow's Champions League game against Benfica.
"What he does for the team is fantastic and to play football for a striker is not just about scoring goals.
"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."
United, top of Group A in the Champions League, with three wins from three, host Benfica at Old Trafford and will qualify for the last 16 if they better CSKA Moscow's result against Basel.
"Ten points would be OK to qualify but our main aim is to finish first in the group," Mourinho said.
"Nobody here thinks about any other match than Benfica. Nobody here thinks about Chelsea," he added, referring to this weekend's Premier League game at his former club.
Mourinho could name forwards Martial and Marcus Rashford in the starting lineup as he did in the League Cup win over Swansea City last week.
"That's no problem," he said. "What we cannot do is play with 12 men, but we did play them together, so we can repeat that against Benfica or we can repeat in any other match."
