Sir Alex Ferguson believes the match against Manchester City on April 30 at the Etihad Stadium is now a Premer League 'decider'.
Ferguson also claimed it was the most important derby of his career at Manchester United after the club dropped two points at home to Everton on Saturday.
United let a two-goal lead slip with only seven minutes remaining on the clock, having led 4-2 after a Wayne Rooney double and goals from Danny Welbeck and Nani. But after late strikes from Nikica Jelavic and Steven Pienaar, the game against City carries even greater significance.
"We've given (Manchester City) the initiative, there's no doubt about that," he told Sky Sports.
"It makes the game at the Etihad a real important game. It's the decider really. We make it hard for ourselves as we normally do. But we have to go there knowing that we're capable of getting a result.
"We need to get a result now at the Etihad, there's no question about that. We need to perform. There's no reason why we can't do that but there's almost an expectancy from City that this could be their decider, but it's our decider too.
"There will be a reaction, alright, there's no question about that. Next Monday against City, it's always a derby game of the highest proportions, and this won't be any different. Yes, [it's] definitely (the most important of my career). Game on."
Ferguson was disheartened by the way his team had defended, gifting some soft goals to the visitors that he said erased the hard work and good football that they had played, with Wayne Rooney and Danny Welbeck's link-up play earning praise.
He continued: "It was a throwaway, an absolute giveaway. We just needed to see the game out, and it's a travesty because some of our football was fantastic.
"The goals we scored were great. To give away four goals at Old Trafford in a home game that's so important…I just can't believe it.
"I think it was defensive lapses. The goals were really soft goals. We've created goals by really good football, but I think [Everton] got their goals easily.
"I think today was a bad performance (defensively). In previous games we've defended very well. It was a real work for us to have defending like that.
"(Welbeck and Rooney) were a real threat today. Their combination play and understanding of (each other's position) was terrific. They should have got more out of the game than they did."