Let me first say that I agree with you that Manny was very good at playing the ball off of the monster and gunning it quickly back into the infield.
Outside of that, the only reason why he was "good enough" was because his offense was so strong.
This is not meant to be an insult to you or the rest of the posters who are claiming that Manny was much better than Hanley defensively, but you guys are exhibiting a classic case of confirmation bias and other things that are wrong with the "eye test".
Hanley was so obviously bad because you could see his bumbles. You could see his misplays. As I said, they were obvious.
Manny was nearly as bad defensively, but his weakness fell mainly with his range, and lack of range is not as glaring or as obvious.
Also, Manny hit while Hanley didn't. The Sox were mostly winning when Manny played LF while they finished in last place with Hanley playing there last year. Those are huge factors in bias for Manny and bias against Hanley.
I know most of you don't have any use for defensive metrics, but years of defensive metrics all saying the same thing, that Manny was a terrible defender, do not lie. Besides that, his terrible defense was usually noticeable just watching the games, even without the stats.