In his last 13 years I think he only hit .300 once, and was little more than a decent hitter who split his time between a number of different positions and played a lot of games as the DH in a Boston lineup with one of the best groups of outfielders of any era (Jim Rice, Fred Lynn and Dwight Evans).
I agree that Yastrzemski should have retired earlier. My main memory of him is when he made the final out in the 1978 tie-breaker playoff game with the NY Yankees with a weak in-field pop up.