United appeared to have found its goalscoring touch during a 4-2 win at Leeds United last weekend.
But it reverted back to what has often been its type under Ralf Rangnick, dominating possession and chances but ultimately disappointing in front of goal.
Cristiano Ronaldo, Bruno Fernandes, Anthony Elanga and Paul Pogba were all guilty of spurning opportunities as struggling Watford clung on to claim a valuable point in a result that will serve as a further boost to United's rivals in the chase for UEFA Champions League football.
United quickly set about an onslaught on the Watford goal and saw a pair of gilt-edged chances go begging as Ronaldo turned and struck the left-hand post from Aaron Wan-Bissaka's cross, and Bruno Fernandes could only shoot straight at Ben Foster after a wonderful move involving Ronaldo and Anthony Elanga.
Ronaldo then had the ball in the net only to be correctly flagged for offside, before Fernandes skewed wide from Paul Pogba's cross with the goal at his mercy.
Fernandes was soon guilty of similar profligacy, heading wide under no pressure after Ronaldo had skipped over Foster and found his compatriot with a cross from the right.
Pogba made a dismal waste of a superb far-post ball as United continued in the ascendancy after the interval.
Ronaldo was aghast not to see Kevin Friend point to the spot after he went down under Hassane Kamara's challenge, and all around Old Trafford were stunned not to see Elanga find the back of the net after he latched on to a deft flick from Pogba.
There was further incredulity when Elanga then inadvertently blocked a goal-bound effort from Ronaldo after excellent work down the left byline from Fernandes.
Jadon Sancho and Marcus Rashford were each thrown into the fray as United sought the breakthrough.
But a deserved winner did not arrive as Watford secured a stalemate that will be cheered heartily in London with Arsenal, Tottenham Hotspur and West Ham United all chasing the Red Devils.