He'll prob end up with 15-20 league goals eventually, but Juventus don't have the FBs Real Madrid got, the CBs Real Madrid got nor the midfielders Real Madrid got.
They don't even have the sort of playmaking striker Real Madrid have with Benzema. Mandzukic is good and will sacrifice himself for the team, but he's just a different type of player CR is used to.
He had the perfect situation in Madrid where he could continue to compete and be among the top 3 favorites for the CL, but his ego got to him.
Not saying he can't win a CL with Juve, but it certainly won't be as easy as with RM.