Manchester United moved to within five points of leaders Manchester City courtesy of the win at Vicarage Road but they had been cruising at the break.
Former Watford winger Ashley Young scored two stunning goals inside six minutes to break a scoring duck stretching back to May 2016 and, when Anthony Martial tucked home a third, the visitors looked home and hosed.
Instead, Watford hit back as substitute Troy Deeney converted a 77th-minute penalty before Abdoulaye Doucoure turned home to bring Marco Silva’s side within one goal.
The points would be secured in style as Jesse Lingard scored a fantastic solo effort after running at the Watford defence, but Mourinho felt his side had taken their foot off the gas having raced into a lead.
“It was an easy match to win comfortably, in the last minutes we should be smoking a cigar, but in the end it is three points,” Mourinho said on BT Sport.
The Portuguese added in his post-match press conference: “We had chances for four, five and six (goals). I think the players had the feeling that it was too easy and the game was over. Then they (Watford) reacted, scored the penalty and that gave them the belief.
“The teams in mid-table have quality and no pressure, they don’t feel the pressure the teams with other objectives have so they came with everything.
“I think we had mistakes in the box and that cost us two goals and maybe it is a good lesson and it is good that we have lessons without losing points, so the next time we are winning 2-0, 3-0…