It was an awful start for the new Chelsea manager in his first league game on the bench for his old club as they struck the woodwork twice in the first half but paid for their defensive frailties.
Tammy Abraham crashed an early drive against the United bar but the hosts got in front with an 18th minute Rashford penalty after the England forward was pulled down by Kurt Zouma.
Chelsea, who had been the more creative in the opening half, went close to an equaliser when Emerson struck the post shortly before the break.
But the game turned decisively in Ole Gunnar Solskjaer’s side’s favour when, after a Rashford led counter-attack, Anthony Martial bundled in a low cross from Andreas Pereira in the 65th.
Two minutes later, Chelsea’s defence was again exposed when Rashford timed his run to perfection to latch on to a long ball from Paul Pogba and beat goalkeeper Kepa Arrizabalaga with a confident finish.
Substitute Daniel James, a close-season signing from Swansea City, then marked his United debut with the fourth, firing into the bottom corner via a deflection after being set-up by Pogba. (Reuters/NAN)