Salah, who has not been at his best since returning from injury last month, was brought on in the 79th minute, just after the Hammers' second goal levelled the game
Having held out for 90 minutes at Anfield last Monday, United conceded the quickest goal in the Premier League this season to Pedro after 30 seconds and were then ripped apart as Gary Cahill (21), Eden Hazard (62) and N'Golo Kante (70) completed the scoring.
Two up at half time, when United replaced Marouane Fellaini with Juan Mata, Chelsea sat back and waited to hit United on the break with Hazard adding a third after a clever one-two and then Kante driving into the area unopposed to score his first goal for the club.