I see him as a high level bench player on a good playoff team. Think Kleber or Looney.
Not about the Celtics or the comparison above.
I just wanted to talk about Kleber for a second. I feel the league has passed him by - not just him but players of his ilk.
To explain this I will described Kleber as a slow footed PF who does not have the size or interior defense to play C regularly. He has good physical size and strength to matchup against old-school PFs as a stretch PF but lacks the quickness to matchup against SFs and combo forwards that now dominate the PF landscape. This puts him at a quickness deficit on defense and leads him to being a net negative on defense.
When we look at his offense, we see that Kleber has a three point shot but he has no handles, no driving capacity, limited passing ability. He offers little to no offense outside of spot up shooting and he only does that at low volume which 7-8pts in 25mpg or so. Something like that.
How much value does that three point shooting offer compared to being a sizeable net negative on defense + a non-contributor to net negative at everything else on offense outside of spot up shooting?
I don't think it offers much at all. I consider Kleber a below average bench player. Possibly even a 3rd stringer.
Then there is the fit issue with Luka. Luka is a slow footed SF who struggles on defense. He can't play with Kleber because Kleber also is a slow-footed forward who struggles on defense. When they play together, they lose more on defense than Dallas gains on offense.
This is why Finney-Smith is so valuable for them - not because he is any great shakes as a player but just because he has enough foot-speed to play alongside Luka for Dallas to have an effective defense.
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This example is interesting to me because players like Kleber looked so valuable just a few years ago but ... I believe the PF position has evolved so much since then that players like Kleber have quickly gone the way of the dinosaur.
The PF position now has so much more speed & athleticism. So much more ball-handling, passing and shot-creation from the perimeter. It has been taken over by more skilled SFs and combo forwards (like Harrison Barnes).
... and those old stretch PFs of previous years who were so valuable for matching up against bigger bodied PFs while stretching the court on offense simply aren't as valuable anymore in today's league.
This is also for our own Grant Williams - not a replica but he is one of those old school undersized non-athletic beefy PFs. Luckily for him, he does have more foot-speed and one-on-one perimeter D functionality than Kleber but there are still a lot of limitations that lead to low value there (questionable team D quickness, lack of offensive diversity).