But with time running out, United's young French starlet Martial hooked Ander Herrera's cross back into the net from the narrowest of angles.
Just like holders Arsenal's exit earlier on Sunday in a 2-1 home defeat by Watford that heaped more pressure on manager Arsene Wenger, a United exit would have raised more questions about the future of boss Louis van Gaal.
With United 2-0 down from the first leg of their Europa League lat 16 tie against Liverpool and 13 points adrift of Leicester City in the Premier League title race, Martial's strike has offered blessed relief for Van Gaal.
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United will now have a replay at what will be the last Cup tie to be staged at West Ham's Boleyn Ground in the Londoners' final season there before they move to the Olympic Stadium.
(Reporting by Ian Chadband; editing by Ken Ferris)
