Lukaku has since failed to score in his last five matches, with apparent fan agitation in Saturday's 1-0 win against Tottenham getting under Mourinho's skin.

The United boss said it was unfair that fans "don't support him so much" as, whether he scores or not, the Belgium international makes a difference - frustrations he repeated ahead of the Champions League clash with Benfica.

"The fans are the fans, and they pay their ticket and are free to express themselves however they want," Mourinho said.

"But my job is, when I feel they deserve, to protect my players and I feel Romelu is one of the players that should be untouchable in terms of the respect from everyone.

"It's not one ball that hit the post or one save by the opposite goalkeeper that can make Romelu's contribution below the top level.

"He's playing extremely well for us and yes, I have to protect my players when I think my players deserve it.

"Romelu always deserves it because of what he does for the team, what he does for the team is fantastic. Playing football for a striker is not just about scoring goals.

"So for me he is untouchable for my team and untouchable also in the support and the respect that he deserves from the fans."

Such sentiments were echoed by Lukaku's team-mate Eric Bailly, who is pleased to be playing alongside the striker having faced him when Lukaku was at Everton.

"I think, like many people, that Romelu is a great forward," the United defender said. "I've played against him and now have the fortune to be playing with him.

"He is very important for us. Everybody can see everything is going well for him and I hope he can continue this scoring run and helping the team as he has been doing."

Bailly returned from injury on Saturday and could feature against Benfica.

United will wrap up a place in the Champions League last 16 if they gain a better result than third-placed CSKA Moscow do against Basel.

Benfica have lost all three Group A matches to date but Mourinho is expecting a tough evening, having edged past them 1-0 after a goalkeeping mistake in Portugal a fortnight ago.

"Our aim is basically to qualify," he said. "Ten points would be OK for qualifying but our main aim is to finish first in the group.

"We know the statistics are what they are but Basel has had good results against CSKA and Benfica. A draw is not bad but Benfica will give us a difficult match.

"They have their own ambition and pride, and they will try to win but we want to get to the last 16 as first in the group."