Jose Mourinho felt his Manchester United side learned a valuable lesson at Vicarage Road as they had enough chances to score eight goals and avoid their late scare.
United did not waste their chance to close the gap to rivals City and are now five points off the Premier League leaders but Watford threatened a comeback.
Two Ashley Young screamers and an Anthony Martial strike gave United a 3-0 lead at half time before Troy Deeney and Abdoulaye Doucoure set up the nervy finish.
A fine Jesse Lingard solo goal in the 86th minute completed the scoreline but Mourinho said: ‘I prefer to forget the five minutes where we conceded the goals and lost concentration.