Guess the Prospect! (1970s Edition)

I was signed as an undrafted free agent out of high school in the early 1970s. An athletic outfielder with power and some speed, I hit very well in rookie ball and even better in my full-season debut in my first full year at age 20. I jumped to Double-A at age 21, but my power disappeared. I then spent the next five years in Double-A and Triple-A, gradually improving my performance but never quite living up to what was expected of me. I didn't receive a legitimate chance in the majors until I was 27 years old, and even then I didn't perform too well. However, at age 28 I figured out what I was doing, and was a productive, solid major league outfielder for the next six years before a slow fade as a platoon player.

Who am I?

The first person to get this right gets to pick the next Career Profile after Chipper Jones, or a Not A Rookie after Jordan Zimmermann.