United boast of nine points from three games and a win at Old Trafford will see them qualify for the knockout stage of the competition. The centre-forward created Anthony Martial's 81st-minute victor and Mourinho also pointed to how the Belgian was hard for Toby Alderweireld and Jan Vertonghen to deal with.
"The fans are the fans and I am also told the fans pay for their tickets and they are free to express themselves the way they want to express themselves", Mourinho said at United's training base.
"They can boo a player that doesn't deserve to be booed, they can boo a player that is working like an animal and because the game is not going in his direction".
"It's not one ball that hit the post or not one save from the opposition goalkeeper that can make Romelu's contribution below the top level". Romelu is one of the players that should be untouchable in terms of the respect of everyone.
"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".
The Belgian worldwide hit the ground running at Old Trafford following his £75m off-season move from Everton, scoring seven goals in his first seven Premier League matches. "They pay the ticket they can do what they want".
The players, and manager Jose Mourinho, looked relaxed as they walked up the steps into their plush stayover.
"Ask Vertonghen and Alderweireld if he was quiet, and they are some of the best centre-backs in football".
"I don't think it is fair at all".
With City having been excellent so far this season, it will certainly be a hard task for United to win the title but they will be determined to capitalise on any slip-ups that their rivals make.
"Whether he's scoring goals or not scoring goals is all that some people analyse. Nobody here thinks about Chelsea", he added, referring to this weekend's Premier League game at his former club.