Green admits the academy graduate made his experienced teammates looks silly in training last season.
"We had Willian, Pedro and obviously Eden [Hazard] in the same positions but the manager had such a strong belief about playing the players he trusted. He would not play other players," Green told BBC Radio 5 live.
"It was the same team, the same formation every week, maybe one or two changes and Callum just wouldn't get a look-in.
We'd go into training and he'd tear it up day in, day out and the things he would try in training with no pressure… and he'd do it in games. You'd just see him trying stuff because he just wanted to.
"It's not that players like this don't care. It's just that they can play with the freedom and understanding that they can try these things and are good enough to pull them off.
"He would try it in training against some of the best players in the world, some of the best defenders.
"Azpilicueta playing up against him in training… couldn't live with him! He'd chop inside him and Azpi would turn around and go, 'Oh God, he's gone past me', and then he'd just bend one in the far top corner or fire one in at the near post."