Leading his side's late surge to the title, the Dutch striker scored 14 goals in United's last nine games to take his tally for the season to 43.
"I think Ruud van Nistelrooy's form has without question been the deciding factor," Ferguson told the Manchester United website on Monday.
"When a centre-forward like that scores the number of goals in the important games that he has, it has to be the difference between the two teams.
"I knew he was going to become a great player. But I didn't know quite how good he was going to become, I must say that, because he has surprised me almost every week."
Ferguson said he was delighted to have put his retirement plans on hold because he enjoyed the enthralling battle with Arsenal.
United, eight points behind Arsenal two months ago, took 45 points from a possible 51 since losing at Middlesbrough on December 26, winning 14 games and drawing three.
"When anyone loses a title, particularly one that was in their (Arsenal's) grasp for a long time, I think it will be a big disappointment to them," Ferguson added.
"But the battle's over. We expect them to be challengers next year and we look forward to that because I think from my own personal situation, I feel that my decision to not retire is the best decision I've probably made."
GREAT TRIUMPH
The United manager also praised his side's character in winning their eighth premier league title.
"It's a fantastic feeling to be champions again. It's been a great triumph by the players, a great effort by them," he said.
"They've shown determination, perseverance and ability. I think they're the qualities that have won it for us...this is the greatest achievement."
Written off by many after an inconsistent start to the season partly caused by a crippling injury list, United hit rock bottom in November when they slumped to a 3-1 defeat at local rivals Manchester City.
But Ferguson's men came storming back to claim the title when Arsenal, who needed to win their last three matches to have a chance of retaining their crown, lost 3-2 at home to Leeds on Sunday.
"To be so many points behind and come from so far off the pace to catch them on the line so to speak, it was marvellous," said Ferguson.
"Our form in recent weeks has been unbelievable.
"Four goals against Liverpool, six against Newcastle, two down at Highbury, three against Blackburn Rovers, two down at Tottenham, four yesterday. Phew, what a run, it's been absolutely magnificent."
