The Red Devils boss this week urged the club’s supporters to stick by the Belgium, despite a dip in form which has seen him fail to score in his last three outings.
However, having started the Premier League campaign with seven goals in as many matches, Mourinho believes Lukaku can be forgiven for not finding the back of the net each and every time he plays.
"The fans paid their ticket, they are free to express themselves the way they want to express," Mourinho said ahead of Manchester United's Champions League match against Benfica on Tuesday.
"But my job is to protect my players and I think Romelu is one of the players that should be untouchable in terms of the respect from everyone.
"It's not one goal or one ball that hit the post that can make Romelu's contribution below the top level, he's playing extremely well for us and I have to protect my players when I feel they deserve it.
"Romelu always deserves it because what he does for the team is fantastic, and playing football for a striker is not just about scoring goals. So, untouchable in my team and also in the support and respect he deserves from the fans."