Juan Mata, one of Jose Mourinho’s five changes from Wednesday’s win over CSKA Moscow, struck early on, netting his first Premier League goal since February on his 200th appearance in the competition.
And two close-range finishes from the in-form Fellaini either side of half-time took his season tally to four before Romelu Lukaku sealed a comprehensive victory late on as United continued to impress even without the injured Paul Pogba.
United’s win, which saw Marcus Rashford also shine with two assists, moved them top of the Premier League, a position they will keep unless Manchester City can defeat fellow title rivals Chelsea at Stamford Bridge later on Saturday.
Mourinho’s side have won seven consecutive matches in all competitions, while Palace are pointless and still without a goal from as many top-flight games as their abysmal start to the campaign goes on.
Palace’s confidence was already shot coming into the game and United took advantage to move ahead inside three minutes.
Rashford – another of the stars brought in by Mourinho – was the creator, easily cruising past Joel Ward down the left and cutting back to Mata, who finished simply with his left foot from 12 yards.