The current gold standard in pre-PBP/hit location defensive metrics (in my near-worthless opinion) is Sean “CHONE” Smith’s Retrosheet-based TotalZone. If you look on his historical WAR (Wins Above Replacement) page for Morgan (http://www.baseballprojection.com/war/m/morgj001.htm), you find that Morgan is -43 runs (below average) for his career in TotalZone, and -4 runs for double play rating. He had some seasons where TZ has him as very good, particularly in the early 70s, and some seasons just as bad.
All defensive metrics at this point have some questions, especially one that isn’t based on specific hit location data (which is the case for TotalZone in Morgan’s era). Still TotalZone’s more recent years usually match up fairly well, overall, with stuff like UZR. I think it does pretty well with infielders in general. And MGL (the inventor of UZR) alsno notes that non-PBP metrics get more accurate over a longer stretch of time.
-47 runs defensively overall might seem pretty bad overall, but that also includes Morgan’s decline years, etc. It’s in 2649 career regular season games, or a bit under 20 full seasons full-time. So that’s more like 2 runs below average per season… And given the error bars for defensive metrics, I think it’s fair to say that overall he was average.
But smarter and more informed people may have different opinions on how to interpret that stuff.