West Ham were anxiously looking for how to get back into the game, but they failed to defend their goalkeeper Joe Hart allowing Anthony Martial and Paul Pogba to score in the 87th and 90th minutes respectively as the match ended 4-0 in favor of Manchester United.
Having survived that late scare, it was business as usual for United after the break as they doubled their lead within just eight minutes of the restart, when Zabaleta was yellow-carded for blocking the escaping Rashford on the right-hand edge of the Hammers area.
And the Armenian made it two assists for the day a few minutes from time as he played in substitute Anthony Martial with a handsome first-time pass. He took so much, mostly unjust, criticism last season.
Mata was through again shortly after following a lovely looped pass over the top by Pogba, but the Spaniard's square pass for Lukaku at the far post was cut out. Mourinho expectedly gives the Old Trafford debut to Romelu Lukaku, spearheading an attacking lineup that includes fringe starters Marcus Rashford and Juan Mata.
And the Argentinian paid a heavy price for that foul for, when Mkhitaryan floated over the consequent free-kick, that man Lukaku ghosted in front of Masuaku to nod home his second goal of the afternoon.
He might have been able to celebrate his first league goal even sooner. By that point, West Ham's heads had already dropped. Great from Pogba, awful from Hart.
The Premier League match between Manchester United and West Ham will be broadcast live online and on NBCSN. Mourinho has to win the league here.