The 37-year-old former England international retired in 2011 only to be coaxed back into the game by Ferguson who made him a key part of the Manchester United side that finished the season as Premier League runners-up.
But Ferguson reckons that nothing compares to the real thing, insisting it will be a massive task to finally replace him.
"A year ago, he was coaching my Reserves team," Ferguson said. "Now he's back playing and everybody attached to United is absolutely delighted because he's such a fantastic player.
"In the long term, trying to replace Paul is a different issue all together. It's going to be very, very difficult. To replace a great player like that would be hard for anyone."