Goals from Nikica Jelavic and Steven Pienaar secured a staggering 4-4 draw for Everton in the latest of a sequence of amazing results this season, which now mean United go to Manchester City knowing if they lose they will surrender the initiative completely.
"Yes, definitely," said Ferguson, when asked if it was the most important derby he has known. "We've given them the initiative, there is no doubt about that. It makes the game at the Etihad a really important game. A decider really."
He added: "There has been an expectancy from City that it could be their decider, but it's our decider too."
Twice in the second half United established two-goal leads after initially falling behind to Jelavic's opener.
Wayne Rooney scored two more goals to take his seasonal tally to 33, one adrift of his best tally, and 190 overall, fourth on United's all-time list.
When Danny Welbeck and Nani scored in the space of two minutes around the hour mark, it appeared victory was secure.
But Ferguson was stunned by the collapse of a team that had kept six clear sheets in its previous seven games.
"There were defensive lapses," he said. "Their goals were really soft. It was a real shock for us to defend like that.
"It was a travesty of a result in some ways but we made it difficult for ourselves and if you look at our history we keep doing that. We've thrown a game away that we should be coasting. But we didn't so we have given ourselves a difficult task."