United could have been left to rue those missed opportunities as Portsmouth came more into the game after the break until substitute Michael carrick sealed the win with a second goal just eight minutes from time.
"In the first half you saw some of the best football you'll see all season," Fergie told MUTV.
"It was absolutely fantastic. But we missed about four or five chances, so instead of going in five-nil up at half time we were one-nil up and made it difficult for ourselves.
"That only encourages your opponents and they kept digging in and got two opportunities in the second half, and I was saying to myself: 'we're going to throw this away.'
"It can be so embarrassing. You're talking about throwing a game away when you should have been five or six up.
"We missed some fantastic opportunities.
"Fortunately Michael Carrick's come on, made a fantastic run and a good finish to kill the game off."