Sir Alex Ferguson highlighted the importance of goal difference in the Premiership title race after Manchester United defeated Tottenham 4-0 at White Hart Lane.
United restored their six-point lead at the top of the table over champions Chelsea but are effectively seven ahead having scored 16 more and conceded one fewer than Jose Mourinho's men.
Even when United stormed into a three-goal lead after the interval, Ferguson was wary against complacency.
'We looked when we went in front we were starting to get careless and casual at the back, you can't do that,' said the United boss.
'Zeros against your name is the name of the game. No goals against in the run-in when you're in the title race is very important to us.'
United went ahead on the stroke of half-time in controversial circumstances as Cristiano Ronaldo did not appear to get touched when he went down under Steed Malbranque's challenge.
The Portugal winger smashed his spot-kick past Paul Robinson to set the Premiership leaders on their way to victory.
'I don't think it was a penalty but Steed Malbranque stuck his foot out and pulled it in again,' said Spurs boss Martin Jol. 'He took advantage of that but I don't know. It didn't help.
'The referee must make a decision, he is a human being. He was standing there with a good view of it.'
The Dutchman added: 'I wouldn't say Ronaldo was diving. It was a difficult situation.'
Ronaldo has raised his game this season despite the jeers from fans for his part in Wayne Rooney's dismissal at the World Cup, and he shone again against Spurs.
After Nemanja Vidic had doubled the lead, Ronaldo set up Paul Scholes for a third, and Ryan Giggs wrapped up victory.
Referee Mark Clattenburg, who was in charge when Pedro Mendes famously 'scored' in the fixture two years ago, was in the thick of the action, aside from awarding Ronaldo a penalty.
United were furious when Henrik Larsson was challenged by Anthony Gardner in the area, while Spurs had a penalty shout of their own when Gary Neville appeared to hold Pascal Chimbonda.
Scholes added: 'The score says everything - 4-0 away from home at a tough place like this. It's a great result for us.
'Going forward we knew we could exploit them with the strength and pace we have up front, so we tried to get the ball forward as much as we can and cause problems.'
United ended with John O'Shea in goal after Edwin van der Sar broke his nose, while Wayne Rooney took a knock and will require assessment before England duty this week.
Spurs midfielder Jermaine Jenas will miss the friendly against Spain after picking up an ankle injury in the Carling Cup defeat to Arsenal last Wednesday.