The 24-year old Belgian midfielder has kicked off the new Premier League season in fantastic form, scoring goals in both the Toffees' wins over Manchester United and Aston Villa, and attracting praise from all over.
Australian international Cahill, now with New York Red Bull, played with Fellaini at Goodison Park since he arrived from Standard Liege for £15million in September 2008 and believes that Moyes has been a major figure in his development.
"The best thing about Felli is he works very hard and is now reaping the rewards of the player the manager has made him," Cahill said.
"David Moyes has to take a lot of credit for that because he buys players and looks what's best for the player and he's getting the best out of Fellaini."
He added: "He's (Fellaini) probably one of the most physical players I've played with as the way he trains and the way he plays and I'm happy for him. It was a great experience playing with Felli and Everton."