Rooney claimed to be disillusioned with the direction the club was taking and wanted to leave because he feared they would not invest in big-name stars.
But United manager Sir Alex Ferguson talked him round and Ferdinand had a strong word in the striker's ear before he signed a lucrative new five-year contract on October 22.
Ferdinand told The Sun: "Rooney staying was a big part of what we achieved this season. There was a sense of relief from the players, the fans, everybody.
"Wayne is one of the best three players in the world, so to have him on our team-sheet is a bonus.
"I spoke to him that week when he was thinking of going.
"You do it with a Manchester United hat on and explain what you think is best for the club and how you want him to stay.
"If he had left it would have been a very big hole to fill.
"I'm not saying it was unfillable because the manager always comes up with something and you trust in his judgment. But, more than anything, Wayne staying sent a signal out to every one of our rivals in the Premier League that the glimmer of hope they had of us falling away had gone."