Eighteen-year-old Marcus Rashford fired United into the lead with a terrific finish in the 54th minute, and much-maligned Belgian forward Marouane Fellaini beat West Ham goalkeeper Darren Randolph in the 67th minute to put the Red Devils 2-0 up.
United had the better of the exchanges in the opening 45 minutes as well as Fellaini, Rashford, Jesse Lingard and Michael Carrick all had opportunities to score, but were denied by Randolph and a solid West Ham defence. Lingard rifled in another powerful shot from distance early in the second half, but it went just wide.
Then they finally got one into the net when Hammers defender Michail Antonio lost possession close to his own penalty area, and Anthony Martial found the young English striker Rashford, who curled the ball past Randolph into the top corner for a classy goal.
West Ham, though, were desperately unlucky not to be awarded a penalty three minutes later when star French front-man Dimitri Payet was brought down in the box by United defender Daley Blind, but referee Roger East waved play on.
And that decision proved costly for the home side as Fellaini bundled the ball in at close-range from Martial’s shot to hand Van Gaal’s team a 2-0 advantage after 67 minutes.
But the Hammers never gave up, and they threw everything into attack in the last quarter as big striker Andy Carroll became more involved. Manager Slaven Bilic finally changed some tactics by urging his wingers to put in crosses for Carroll and co, and it worked a treat as they went close on several occasions from corners.
The pressure eventually paid off when Carroll flicked on a corner at the near-post, and James Tomkins bravely dived in to head home in the 79th minute and take the score to 2-1.
United held on desperately, and there were two late flutters when Carroll headed over the top, and then De Gea made two back-to-back saves before Cheikhou Kouyate strayed offside as he headed past the Spanish goalkeeper, and was correctly flagged by the assistant referee.
But all that matters for the under-fire Van Gaal is that his team are through to the semi-finals, where they will take on Everton at Wembley on April 23, while Crystal Palace take on Watford at the same venue a day later.
So Van Gaal and United will fancy their chances of clinching a 12th FA Cup trophy, which would draw them level with Arsenal for most titles won.
Original source: Fellaini sends United into semis