Well.. This season since his ban he hasnt been himself but before the ban I`d say he is right and D.Costa was indeed our 2nd best attacking player!
Right now I`d say things have changed and its like this:
CR7 Dybala Mandzukic D.Costa Bernardeschi (by this order)
CR7 will remain the best regardless of what happens
Dybala can remain 2nd if he continues scoring and moving the way he did lately..
Mandzukic is Mandzukic and he is a valuable player for our team
D.Costa when on form is 2nd to Ronaldo (until Dybala finds what we all are waiting for him to find)
Bernardeschi is in a growth process so today he is 5th but it could change in few months time...