Everyone has ups and downs, I've seen blue chips "plateau" or "regress" as part of the course of their careers. Ups and downs on a constant upward trajectory from freshman to senior.
I probably should have said "part of the process or learning curve" as to not confuse the two sayings. Since the term is "course of a career" instead of "process of career" that's where my line of thinking went and hence the term, however, I am familiar with "par for the course."