The veteran midfielder came out of retirement in January to be a key player in the Red Devils' near-miss with the Premier League title last season and Ferguson knows just how hard it will be to employ an apt replacement once he calls time on his career.
"A year ago, he was coaching my Reserves team," Ferguson told the club's official website.
"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."