A great deal depends on who the core players of the contender are. Are they build around LeBron? Kawhi? Jokic? Lillard?
In a vacuum
Tatum - 34-38 mpg (star scorer)
Brown - 34-38 mpg (star secondary scorer)
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Kemba - 24-28 mpg (veteran starter who plays limited minutes; 4th option who occasionally does more)
Evan Fournier - 20-28 mpg (either a scoring reserve or a 4th / 5th starter)
Smart - 24-28 mpg (swiss army knife / energy bench guy)
Rob Williams - 20-24 mpg (part of a center platoon)
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Pritchard - 10-20 mpg (backup point)
Tristan Thompson - 10-16 mpg (back up center)
Romeo Langford - 10-15 mpg (if healthy he should be an athletic reserve / defensive 4th wing)
Luke Kornet - 5-10 mpg (4th or 5th big; stretch option)
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Grant Williams - 0-15 mpg (5th big / forward)
Semi Ojeleye - 0-10 mpg (5th wing)
Mo Wagner - 0-10 mpg (energy end of bench guy)
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Aaron Nesmith - G League (raw project)
Carsen Edwards - G League or overseas
Tre Waters - G League or overseas
Tacko Fall - overseas
Basically as I see it the Celts have:
- two stars,
- four solid rotation pieces,
- another four guys who should see limited minutes depending on matchups,
- a few guys who should be at the end of the bench and mostly not play
- and four guys who aren't really NBA players right now.
It's the fit and health of the top 6 guys, plus a total lack of a middle of the rotation (i.e. 2-3 guys you can count on to play 15-20 mpg), that I see as the problem.