The 20-year-old failed to initially make an impact at Upton Park following a £650,000 move from Manchester United in January 2012, but excelled in the Championship - scoring three times in 27 appearances.
Morrison garnered a reputation as a talented-yet-flawed troublemaker at Old Trafford after a number of run-ins with police, but Clark feels the youngster has now matured.
"I didn’t do anything on the football side, he’s an unbelievably talented young man," Clark told talkSPORT.
"I basically told him it was all about the full package, what he was doing off the pitch as well as on it.
"It’s about behaving in the right way, having a good attitude and that if you have discipline off the field it becomes second nature on the field.
Morrison has scored four times for West Ham this term, including an early goal-of-the-season contender against rivals Tottenham on Sunday.
Gareth Southgate has included the youngster in his first England Under-21s squad, and Clark has backed Morrison to continue impressing.
"West Ham have been absolutely delighted with his response since coming back, he hasn’t given them a moments problem," Clark added.
"It was never about his attitude on the pitch or his work ethic, he loves the game and he was one of the fittest players at our club.
"He’s got his first call-up to England’s Under-21’s this week, so if he can continue in the same vain and perform at that level, I think England will start to consider him, he can change a game in a flash.
"He was unplayable at times for us, he can go as far as he wants to go."