The matches ended in a 4-3 win against Hull at Old Trafford.
"We’re a bit low after that performance," Sir Alex told MUTV. "We didn’t take our chances and that’s annoying. We should have scored 10 or 11 but we frittered our chances away and we’ve almost been left embarrassed.
"The only positive from today is that we’re creating chances. We’re playing some fantastic football at times.
"Ronaldo could have scored four or five. I don’t know how many chances Berbatov made today but we could have scored a lot of goals."
Sir Alex also took a swipe at the referee.
He added: "The deciding factor may have been when Michael Carrick was brought down on the edge of the box and [Michael Turner] didn’t get a booking.
"He’d already been booked so the referee failed in his duty there. It should have been a red card because he was right through. Even a yellow card and the player would have been off.
"And that makes a difference, you know. From there on, we missed all our chances, they’ve come back to 4-3 and we’re left scrambling to get a victory."