Gary Neville says losing the title to Manchester City on goal difference would be Sir Alex Ferguson's "worst nightmare".
The Sky Sports expert and former United full-back was speaking after watching Roberto Mancini's side win the Manchester derby 1-0 to go back to the top of the Premier League.
The local rivals are locked together on 83 points, but City boast a superior goal difference of eight goals and just need to match United's final two results to claim their first league title since 1968.
And Neville says United will rue a series of missed chances throughout the season if things don't go their way.
"I've been in that changing room for 20 years and it's his worst nightmare," he said on Monday Night Football.
"Losing the championship on goal difference is what he always talks about. Every single season in January, February and March in the championship run-in he will say if you're 1-0 or 2-0 up he wants you to score three or score four. You never know whether it's going to go to goal difference.
"You look at games like Fulham at home (1-0) or QPR at home (2-0), who had six put past them [against Chelsea]. It's these things you look back on during a season and think you could have done more.
"It is his worst nightmare."
United failed to muster a single shot on target at the Etihad Stadium - the first time that has happened since May 2009 when they drew 0-0 at home to Arsenal.
And Neville was alarmed by their lack of quality and creativity in such an important game.
"Nothing happened," he said.
"When you see a Manchester United team going forward they always create chances, no matter what game they're playing in.
"I think it's three or four years since they haven't had a shot on target, but they never tested Joe Hart all night.
"They had the players on the pitch in the end. Young, Valencia, Giggs, Rooney, Welbeck; the players are there, but where was the quality?
"There was an Ashley Young corner at the end that hit the first man and a Phil Jones pass when he tried to slip it down the side and it went out of play. Those sorts of things are so frustrating.
"You only need one moment to go your way and they're back in the title race and they couldn't create a single opportunity during the game."
United boasted an eight-point lead three weeks ago, but a 1-0 loss at Wigan and a 4-4 draw with Everton gave City the opportunity to return to the summit on Monday night.
After the game, Mancini said United had an easier run-in - against Swansea and Sunderland - but Neville believes City are now on track to take title.
He says that if they can take three points from Newcastle on Sunday then there's no way they can be caught.
"I think that City are favourites and I don't believe what Roberto Mancini has said," he added.
"When you get into a position where you know one game can win you the league, which City are in on Sunday, I can't believe Manchester United and Manchester City are going to keep passing up these opportunities.
"United have passed up an opportunity tonight because they could have won the league in effect, although they obviously would have needed a draw against Swansea.
"They were eight points clear. Is there going to be another swing? It's been the craziest season you could ever imagine and you can't predict anything.
"But having been there before and won championships if you have one game to win the league, more often than not you do what you need to do."
City host relegation-threatened QPR on the final day, but Neville can't see Mark Hughes springing a surprise against his former club at the Etihad Stadium.
And he says that Sunday's game - live on Sky Sports 1 HD - is effectively a championship match for City.
"Newcastle is a tough place to play and they're still playing for fourth place," he said.
"It is going to be difficult and anything can happen because this season has been absolutely mad.
"If you'd asked Roberto Mancini three weeks ago after the Arsenal game he'd have said they were done. Sir Alex Ferguson has rightly said it's slipped away, but all you can do in football is keep going.
"For Manchester City they have one game. I think the QPR game, with all due respect to them after the way they played on Sunday against Chelsea, are a non-entity.
"It's that game away at Newcastle. Win one match to win a championship.
"Now all the pressure is back on City."