United slipped five points behind champions Chelsea in the Premier League at the weekend following a goalless game at Sunderland.
But it is the points dropped in draws at Fulham, Everton and Bolton that have most irked their manager.
Speaking to the club's official magazine, Inside United, Ferguson said: "This present team doesn't kill off sides as well as they should.
"At Goodison, we had three or four chances at 3-1 to really put the game out of sight, but we were wasteful and ultimately paid the price.
"We threw away two goals in injury-time. That's unthinkable.
"That killer instinct just isn't there at the moment and it's something we need to develop.
"We haven't had a problem scoring goals.
"We got three against Liverpool, Everton and West Ham, two away at Fulham, three in the Community Shield; that suggests we don't have difficulty finding the net.
"What we're not doing, though, is killing teams off.
"I have no doubt the players have taken these setbacks on board. They suffer just as much - if not more - than the rest of us.
"They're the ones throwing the points away and they're the ones who know fine well that they're good enough to win this league."
United have been criticised for some of their performances this season but Ferguson said: "We're playing well, and perhaps those four points we have dropped away from home at Fulham and Everton have given off something of a false impression.
"Nevertheless, we didn't like letting those points slip away and it's crucial that we don't make too many more similar mistakes this season."
The United boss also kicked off the mind games with Chelsea by claiming that they have had such an easy start, they should have won every game this season.
The Blues have, in fact, taken maximum points in all but one match, their defeat at Manchester City.
Asked if United could not afford to drop any more points, Ferguson said: "No, no, no. Every team is going to drop points - that's inevitable.
"If you look at Chelsea's start to the season, you'd have expected them to have won all those games, so the position they're in doesn't surprise me.
"Meanwhile, we've had tricky trips to Fulham and Everton then played Liverpool at home. Those are difficult matches.
"Liverpool, Arsenal and ourselves have all played top sides early on.
"Chelsea's sterner tests are still to come."