But to your point about his future being more at C instead of PF, I don't think that is a reason to do or not do a deal for him now. Horford is likely gone after this season so there will be opportunity to play him wherever you want or where he plays best. If we get him, the thinking should be long term, not short term.
If you think he is a real deal big, regardless of PF or C, you try to get him. Do you think he is a good fit alongside Brown and Tatum? If so, you can work out the line ups.
(1) Definitely. Long term fit is the priority. I would view him as the long term center and then look to gradually reshape the team around him. So I'm not put off by the roster not being ideal for Markkanen right now. That is a non-issue for me.
We can change the roster and get it right for him whether it that happens early this season, midseason or next season. At some point, we'll get it right to get the maximum out of him. Otherwise, I would not sign him.
(2) Is he a good fit alongside Jaylen and Tatum?
Not particularly. No.
I feel the best fits with Markkanen would be ball-handling playmaking perimeter players. Players who can manipulate a defense with their passing and ball-handling to best exploit the spacing and matchup issues that Markkanen presents.
I see Tatum and Jaylen more as scoring forwards. And due to their scoring prowess, I'm not sure they benefit much from a player like Markkanen. They may be better off with a more defensive minded center who grabs lots of rebounds than a high scoring jump-shooting center. Or as well off on offense with a more limited spot up shooting center like M.Turner or B.Lopez.
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I do like them as a quick-footed frontcourt trio:
F: Jaylen
F: Tatum
C: Markkanen
But that still leaves a lot of pressure on the guards to be able to run an uptempo run and gun offense which means you probably need a lot of cash leftover to sign good enough guards to make the most of that frontcourt. Doable but not easy. And much more difficult if Markkanen is indeed a big success as a high scoring center and warrants max money after this initial shorter contract expires.
Overall, I see a passer / scorer as a better fit as a third star alongside Jaylen and Tatum than just as a scorer like Markkanen is because that leaves big offensive gaps that still need to be filled.
Or at least provide enough high end defense or rebounding alongside the scoring as a 3rd star ... which Markkanen also does not provide.
So I do not view him as a particularly good fit. Not awful either, more mediocre.