Upon further investigation, I think I've been confusing Howell with Wayne Embry all these years. I knew Howell's stats but Emrby, for whatever reason, is the guy I've had in my head.
Embry wasn't a very big part of the Celtics but he was on the championship team in '68 had made a handful of all star games for the Cincinnati Royals in the early 60's. He was only with the Celtics for 2 seasons. Howell was in Boston for 4 seasons, 3 times averaging over 19 ppg and twice for championship teams. Looking over his numbers from those first 3 seasons in green, he completed a pretty devastating scoring trifecta with John Havlicek and Sam Jones.