Defensively no one was worse than the late-middle career Jackson. I'll give you that.
I recommend you pick up Bill James' "Historical Baseball Abstract", printed about 20 years ago for an excellent discussion of the mathematical science of baseball, and myriad hilarious anecdotes.
In 1930 the ENTIRE NL hit for a .303 average because the ball was juiced; does that mean that all of those players were better than todays?. Of course not. The only way to compare players across eras (in my humble opinion) is to compare them to their contemporaries. Ex.: How many years was Jackson in the top 5 AL HR hitters? How does Sheffield compare in that regard? Do the same with batting avg., slugging pct., on-base, etc., and you'll figure out which one is a better overall player mighty quickly.
For example, we know without a scintilla of a doubt that Lefty Grove was the greatest pitcher of all time. How? a. He played his entire career as a lefty in the two worst parks for lefties in the AL (Shibe & Fenway), yet he has the highest lifetime winning pct. of any 300-game winner. b. (And more significantly), he won the league ERA title something like 8 times in those same bandbox parks - about 3 more than the next closest fella - whose name was Walter Johnson.
Incidentally, Bill James has been an advisor for the Red Sox for a number of years, and they...well...any questions?