Sir Alex Ferguson admits Manchester United already face an uphill battle to retain their Premier League title after their derby defeat by Manchester City.
Ferguson endured a highly frustrating afternoon at Eastlands
United are five points adrift of main rivals Chelsea after claiming just two points from their first three games.
"We have given ourselves an uphill fight, there is no doubt about that," said Ferguson after Sunday's 1-0 loss.
"We have to get a win under our belts. We have Tottenham on Sunday. That has become a very important game for us."
United have made their worst start since 1992, when they drew one and lost two of their first three games, before recovering to go on and win the inaugural Premier League crown.
The champions have dominated large parts of all three of their opening games, but failed to convert a catalogue of chances, registering just one goal to date.
We have to make sure we do not get carried away with a knee-jerk reaction
Cristiano Ronaldo's suspension for his red card against Portsmouth has compounded the absence of strikers Wayne Rooney, Louis Saha and Ole Gunnar Solskjaer through injury.
"We had enough chances and played fantastic football at times," Ferguson added of Sunday's defeat.
"We were just wasteful, we had so many opportunities to win it ourselves.
"I can't fault the performance - we completely dominated. But we should have sewn the game up.
"Of course you miss strikers but the number of chances we had and the amount of possession we should have won the game."
But Ferguson insisted there will be no panic despite his team's frustrating start.
"At this moment in time we have to rely on our experience and make sure we do not get carried away with a knee-jerk reaction," he added.