Jose Mourinho’s team were a constant threat against a poor Hammers side, offering excitement and encouragement to their supporters on the opening weekend of the new Premier League season.
Lukaku, a £75m summer signing from Everton, put the Red Devils ahead with a rising drive off the post after racing onto Marcus Rashford’s pass.
The Belgium international doubled the lead after half-time, glancing in a header from Henrikh Mkhitaryan’s inswinging free-kick.
United teenager Rashford curled against the inside of Joe Hart’s left-hand post before substitute Anthony Martial ended the match as a contest with a slick finish.
There was still time for a fourth, however, as Paul Pogba curled in from distance.
While happy with the win and performance, Jose Mourinho was not getting carried away after one victory saying:
‘We start the second season with a very good performance at home,’ said Mourinho.
‘We had many players on the pitch with a very good game and we have reasons to be optimistic.
‘For me, years and years of experience and experience in the Premier League, my feet are on the ground and I am calm.
‘Last season we were also top of the league in the first match (a 3-1 win at Bournemouth) and we finished sixth so this means nothing.
‘It just means we played well and confidence levels will be high and now our challenge is to keep these confidence levels. I think we can do better than last season.’
Mourinho will also make sure to remind his players of the mistakes they made in the game:
“I am but I remember every second of the game. I don’t forget the ball hitting the bar at 2-0 and Diafra Sakho coming onto the pitch.
“In spite of speaking about Sakho at half-time, he was free and almost scored.
“If the score was 2-1, the last 20 minutes, instead of enjoying trying to score more goals, it would have been about not conceding the equaliser, for not losing two points.
“Yes, we did very good things but the thing I’m more pleased about is the confidence levels in the team and individually. The confidence to play good football.”