The Belgian striker made a dreadful mess of a chance to put United 4-0 ahead, taking an age to shoot with the goal at his mercy before being closed down.
It was the most glaring miss on a night in which he still contributed with some sterling work as United closed the gap on leaders Manchester City to five points.
Mourinho was in playful mood when discussing Lukaku, saying the 75 million pound striker's lack of goals of late could be put down to his lack of a boot endorsement deal.
"I think he needs a big contract with boots because at this moment he doesn't have a contract with any brand, that's why he's playing with black boots!" Mourinho told reporters.
"He finished his deal and now he is waiting for the right offer and he is playing in black boots.
"I think he needs a brand to give him the right boots and to pay him the right money so he goes back to goals!"
While Mourinho could afford a smile, thanks to two goals from Ashley Young and one from Anthony Martial that gave his side a 3-0 half-time lead, then a superb fourth from Jesse Lingard after Watford threatened a comeback, Lukaku's struggles are no laughing matter.
After joining from Everton he scored 11 goals in his first 10 games but has now managed just one in his last 11.
Yet Mourinho appears unconcerned.
"Jokes apart, he works amazingly well for the team. I would never blame him for missing an easy chance, never.
"He works amazingly well. Amazing player, amazing professional, great condition, great colleague," Mourinho said.
"I couldn't be happier with him."
United's tally of 32 points from 14 games is their best since the 2012-13 season when Alex Ferguson's side went on to win the tile in the Scot's final season at the helm.