I agree Patrick Yates in that I think Tatum has cemented his stay with the C's. Danny Ainge is no dummy and he made it a point to trade the #1 pick in the 2017 draft because he was really big on Tatum. In regards to Kyrie and Heyward being worried, I don't think they have anything to worry about. Irving and Heyward are Superstars and will be starters once they return. The players whose time on court reduces are Rozier, Morris, and Ojeleye. I would not be surprised to see teams lacking PG play to make a move for Rozier after the Playoff performances he has put on this season.
As well as he's played, and as often as Kyrie is injured, I'd retain Rozier.* I think the market for Smart will be brisk via sign and trade, as I feel a lot of teams will view him as an excellent guard to add to the rotation. Losing his D will hurt, but the Celts have to start making some hard decisions.
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Ainge is currently basking in the glow of building a team that looks to be on the precipice of greatness. No matter what anyone ever says, it is the draft picks that best reflect on the GM. He's already being lauded for drafting Brown (higher than many projections said he should go), and the brilliant-in-retrospect trade down for Tatum.
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Seriously, those could be the type of draft picks that are mentioned first when he accepts GM of the year awards in the future. Just brilliant draft moves. He won't be keen to burn that down.
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As I said, in 2-3 years, the Eastern Conf finals, between Boston and the Celts (how erect are the execs in the league office and at ESPN/TNT for that to happen?) will be the de facto NBA title series. He's not dumb enough to squander what could be a decade long time in the sun, with a few series vs the LAL probably in the middle (now those league and TV execs need new trousers/undies/chairs). That is how a GM becomes a legend.
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And, an oft injured Embiid, whom I personally don't think will put in the off season work to really prepare, is surmountable. The Phillies will be good, and will take a few of those series vs the Celts, but if the Celts make the correct moves, the future is bright.
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A possible road bump is the relationship between Stevens and Heyward. In 2 years, contracts are going to start coming due for renegotiation. Based on future value (and a lowered cap), and current value, Heyward will represent the most value via trade. He could be turned into the type of valuable backups that title teams must have. Will Stevens be OK with jettisoning a guy whom Stevens used his personal relationship with to lure to the Celts? Like I said, only possible fly in the ointment.
*Barring sweetheart trade offer. If I'm the Celts, only Brown and Tatum (in that order) are untouchable. I'd include Horford, but given his age I'm not sure reasonable compensation would ever be offered. Horford is very important, but I don't think anyone will offer a decent trade for him, so he's ESSENTIALLY untouchable. Everyone else, Kyrie and Heyward included, would be available for the correct offer.