Duff's dribbling ability and penetration had been sorely missed by Chelsea since he dislocated his shoulder at Fulham on December 20.
However, the Italian's 84-year-old mother will have been encouraged to learn that her favourite player was back in action as a substitute in last night's FA Cup third round replay against Watford.
Ranieri declared: "I'm very happy to have him back.
"He's a fantastic player and when he goes with the ball one on one against an opponent, he is devastating. He is an intelligent, smart player.
"When I call my mother, she will say 'well done, son' for bringing him back."
Ranieri admits he also faces a considerable battle to convince his mother not to believe all the transfer rumours swirling around Stamford Bridge as the club continues to be linked with Louis Saha, Christian Vieri and Scott Parker.
"My mother is worse than the Press!
"Every day, she asks me about transfers and believes everything she reads and watches not what her son tells her," he admitted.
Ranieri has, however, publicly confirmed his admiration for Parker, the Charlton midfielder for whom a stg£6million bid was turned down earlier this week.
Charlton yesterday dismissed reports of an improved stg£10million offer from the Blues, with the club reiterating chief executive Peter Varney's comments earlier this week.
Varney had insisted: "Scott's not for sale and we do not welcome this inquiry. We sincerely hope that Chelsea accept our decision in this matter."
Not that Charlton may be able to hold out forever in the face of such financial pressure, however, as Ranieri seems keen to strengthen his central midfield.
In an interview earlier this week on Chelsea TV, he admitted: "I can only say that Scott Parker is a good midfielder.
"We are always looking at good players. We don't need anyone I have 21 players. But I want to look around England and other countries to see if there are some other good players."
Asked if Charlton could soon expect a second bid, Ranieri insisted: "I don't want to speak about this.
"But I have a big vision about good, young players. That is my philosophy. If there are good players out there, I want to know about them."
Parker is certainly one of Chelsea's main targets, especially as Juan Sebastian Veron and Emmanuel Petit are sidelined by injury, while Geremi has left for the African Nations' Cup, although Ranieri dismissed talk of a swap deal involving Joe Cole.