United endured a miserable start to the season, losing two of their first four matches and slipping well off the searing pace set by Liverpool, Chelsea and Watford themselves.
But two goals in three first-half minutes from in-form Romelu Lukaku and Chris Smalling gave them valuable breathing space at half-time and ultimately paved the way to a victory that ended Watford’s perfect record.
Rocked by United’s strong finish to the first half, Watford pulled a goal back midway through the second period through Andre Gray but failed to find an equaliser despite a strong finish.
Mourinho hailed the “very important” victory but said his side’s performance level dipped in the second half.
“With this win we keep the same points as Spurs and Arsenal,” he told the BBC. “We have that distance of four and six points to the other top teams. if we don’t win this match obviously that distance goes a little more.”
“In the second half we decreased our intensity, then the goal happened, then the game is open,” he added. “I think this last Romelu Lukaku tackle as a right-back shows everything. I think he is the image of the team.”
– United power –
Mourinho opted for power rather than guile at Vicarage Road with a physically imposing midfield trio of Paul Pogba, Marouane Fellaini and Nemanja Matic.
Former Watford player Ashley Young came in at left-back to replace Luke Shaw, who suffered a head injury while on England duty last week.
The first half an hour was even as the two sides tested each other out –- United probed but the home side looked dangerous on the break, confidence coursing through the players on the back of four straight wins.
Watford went close to breaking the deadlock when Troy Deeney forced David de Gea to tip over just after the quarter-hour mark.
Midway through the half Alexis Sanchez had the ball in the net but it was ruled out for offside, and minutes later the Chilean forced a fine save from Watford goalkeeper Ben Foster following a jinking run.