"I didn't expect that, that's for sure," Sir Alex told MUTV. "But if you analyse why we've lost, the goals we've given away were too soft. You just can't give goals away like that at this level."
United's defence contained a 20-year-old (Fabio), a 21-year-old (Chris Smalling), and a 22-year-old (Jonny Evans), and Sir Alex believes they were given a lesson they won't ever forget.
"They're all young players," he sais. "We've invested in youth, we've done that well over the years, and they will come on from this. They will remember tonight. It's unfortunate that it was a quarter-final of the cup and we had a good opportunity of winning it again this year."
United had started promisingly enough in both halves, but soon succumbed to West Ham's more physical and cohesive team play. Strangely enough, it was a disallowed goal in the first half that appeared to turn the game in the Hammers' favour.
"Possession-wise we had had the majority of it and we had one or two half-chances and one good opportunity when Obertan's shot was turned onto the post. That was unlucky," added the boss. "We were in control of it early in the match and played some good football. I think what changed the game was getting the goal disallowed. It was a break for them. It got the crowd up. But the goals we conceded were poor, and some of our players were too young to carry those mistakes and still win the match."