The veteran insisted United needs to tighten things up and that its seven-point lead over Manchester City "means nothing" as the rivals overturned a similar deficit last year.
The club's 4-3 victory over Newcastle on Boxing Day moved United seven points ahead of rival Manchester City at the top of the table, but there was reason for concern after United allowed the Magpies to lead three times before Chicharito's last-minute winner.
At the mid-way point in the season, Sir Alex Ferguson's side has let in just five fewer goals than it conceded throughout all of last season (33).
Ferdinand acknowledged that the defensive form is not good enough and insisted that the players must "sort [themselves] out" to make sure Roberto Mancini's men do not catch them.
"When you're conceding goals at any level, you want to stop it,” Ferdinand told reporters. "We've let in far too many goals and need to change that. We have to make a big effort to push on from a good position. If we're going to maintain where we are, we have to sort ourselves out.
"We seem to make sure the punters get their money's worth when they come to the ground at the moment. But it was a great show of character from the lads at the start of a really busy schedule. It was really important to get the win, no matter how we did it."
While United pulled off a last-gasp victory at Old Trafford on Wednesday, City lost 1-0 away at Sunderland for the second successive campaign, which allowed Ferguson's men to extend their lead.
Even so, Ferdinand stated that the battle for the league crown is by no means over, demonstrated by last season when Mancini's men clawed back an eight-point deficit with six games remaining to win their first league title in 44 years.
"A seven-point lead means nothing, because we've been in positions over the years of being eight, nine, 10 or even 11 points clear and not gone on to win the title," Ferdinand said. "We've also been behind and won it. It's a great result for us, other results also went for us, but we've got more big games coming up."
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