Romelu Lukaku is the fourth Manchester United player to score a brace on his Premier League debut.
Romelu Lukaku announced his arrival at Old Trafford by scoring twice on his Premier League debut for Manchester United as they embarrassed West Ham.
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.
The Hammers offered little going forward in a passive performance, although new signing Marko Arnautovic did flick a header onto the top of the home crossbar at 2-0.
The Red Devils have now lost only one of their 14 opening-day Premier League fixtures at Old Trafford, while West Ham's season began with a record 11th defeat.