Agree with both--that he is among the top 5 defensive centerfielders in MLB and that he is not the greatest centerfielder in a generation or whatever. I think those who are saying he's the greatest do so to justify keeping him in the lineup despite his horrible slump.
In 2014 I was very enthusiastic about JBJ as our starting rookie centerfielder because I thought Ellsbury was way overpriced and the Sox were right to let the Yankees steal him away. But he ended up in Pawtucket because he could not hit MLB pitching, and that continued until he was called back again in August 2015 and suddenly blossomed as a hitter, which continued all the way through the 2016 season and sort of (his OPS went below .750) in 2017. For those 2 2/3 seasons I thought we had a very good all around centerfielder who was an especially good defender.
This year I expected the same, and I'll bet the Sox FO and new manager did too. For roughly the first 20 games, JBJ was good enough at the plate to keep his regular job, but then three things happened.
The Sox incredible winning streaks stopped and the Yankees went on their own tear and caught and passed the Sox for the best record in MLB.
At the same time, the pitching dropped off a tad, the hitting became that more important, and the bottom third of the lineup became more and more of a drag on the offense.
JBJ became the most expendable guy in the bottom third because Nunez was still our best bat at 2B, Leon/Vazquez were the only viable options at catcher, and JBJ went into an even worse slump.