England striker Rooney said he wanted to leave Old Trafford last October, but then performed a U-turn and signed a new five-year contract.
United will pick up the English league trophy for a record 19th time on Sunday, after their game against relegation-threatened Blackpool, before taking on Barcelona in the Champions League final at Wembley next weekend.
And Ferdinand reckons Rooney's decision seven months ago was a crucial factor in their achievements this term.
The 32-year-old said in 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 judgement. 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."
Ferdinand also claimed Javier Hernandez, who has scored 20 goals in all competitions so far in his debut season with Sir Alex Ferguson's side, should have been named player of the year.
Tottenham winger Gareth Bale was named Professional Footballers' Association Player of the Year with West Ham midfielder Scott Parker handed the Football Writers' Association accolade.
"Gareth Bale and Scott Parker both had good seasons but for me 'Chicharito' has been the main man," Ferdinand said.
"This season it seems a couple of big performances in a couple of games has seen you get championed as player of the year but Chicharito has done the business all season."