Lukaku, who netted 11 goals in his first 10 United games following the move from Everton in the close season, disappointed on Saturday when United laboured to a 1-0 home win over Brighton.
It was believed Lukaku would face retrospective punishment for his actions.
But Omnisport understands Lukaku will face no further action from the Football Association in relation to the incident after it was reviewed by three former match officials.
Till date, the Belgian has scored 12 goals in 20 games but has only managed one goal in in his last 10 appearances.
The 54-year-old United manager added that, in the last minute of the match, Lukaku, "was fighting and running back like he was in the first minute".
"The way Romelu wins the corner for the goal, the way he ends the game making tackles in the left-back position, that's the mentality I want".
Fortunately for him, he will get another chance today, should he play against Watford.
Watford, Arsenal and Manchester City must all be asking if there are other reasons behind the leniency shown to the country's biggest club. "Didn't score, I don't care".
Mourinho is unconcerned about such a drought, however, praising Lukaku's work ethic after newly-promoted Brighton provided what he described as probably the toughest test his second-place team have had all season.