I think Hollinger is somewhat of a slave to PER and his models. And that's okay, because he made them and he's getting paid because of them. But PER can't account for things that aren't publicly tracked. For instance, if I had the time, I'd love to see the fg% of shots that come off of Rondo passes, vs. other's passes, vs. the dribble. My gut instinct says that Rondo is amazing at creating better shots for people, but I don't know. Maybe he just gets a lot of assists because he passes the ball a lot. It's not tracked.
Similarly, there may be things that Jeff Green does which the Celtics place value on, but do not show up on a statline. Furthermore, they may see things that Jeff Green does well under certain situations, and they have the ability to put him in those situations better than the Thunder did, and better than they did mid-season a couple years ago. Perhaps those situations are unique to the Celtics, or perhaps Jeff Green being in the wrong situation was unique to his time in OKC, since Kevin Durant was a similar but much better player, and thus was not complimentary to Green. If the Celtics are unique in their ability to utilize Green, then they did indeed overpay. If the Celtics are not unique, then they may not have. Unless he performs well, is traded, and continues to perform well, we'll never know if they didn't overpay. If he doesn't perform well, we'll know they did overpay.
Also, Hollinger's model probably hates Jeff Green, because he was an average player, and he missed a year, which the model will assume was a serious injury with a lengthy recovery time. In basketball, these are typically ACL tears, which rob players of effectiveness for multiple seasons. Assuming that his heart ailment was truly a one-shot deal, he won't have problems with speed, agility, and strength, like those recovering from ACL's would, but just stamina, which will be mitigated due to his reserve role.
In what (admittedly little since he was paywalled) I've read from Hollinger, I've never seen him point out why his model might be wrong at times. That bothers me. It may or may not be wrong about Jeff Green, but it is certainly imperfect, and he needs to articulate those imperfections better.