The victory is welcome relief for under-pressure United manager Louis van Gaal, whose team has stuttered all season and remains outside the top four of the Premier League.In the last cup tie at the Boleyn ground before West Ham moves to the Olympic Stadium, Rashford opened the scoring in the 54th minute, curling the ball past defender James Tomkins and goalkeeper Darren Randolph.
Fellaini doubled the lead in the 67th minute from close range, before Tomkins headed one back in the 79th minute.
United will meet Everton in the last four on April 23 at Wembley Stadium.
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