It's their funeral, when they take one of the best defensive guards in the NBA? With all due respect, but I don't really understand how you come to this conclusion.
Marcus Smart is no star. Maybe you expect that from a lottery pick. But he is a decent playmaker, a great defender and most of all he is absolutely tenacious out on the floor (and a great actor btw.
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I understand, that shooting is what makes and breaks NBA careers nowadays and Smart is really lacking in that departement. But from my point of view shooting is only one part of the game and Smart does so many little things so well, that he is a positive asset on the floor despite his shooting (btw. he shot 39% from 3 in this years playoffs, so maybe there is still hope he will improve to a point, where he isn't a liability when left open).
Besides his shooting the main criticism of Smart would be his lack of explosiveness, which limits his ability to finish above the rim. That's something, that will always be a weakness (when compared to the freak athletes playing his position), no matter the work he puts in. But like with Zbo or Boogie the lack of explosiveness comes with a sturdy body, that gives him some advantages in other areas. I can't name another guy around 6'4, that can defend bigger guys as well as Marcus Smart. He can easily play SF if necessary or switch on PF's without requiring a double team. That's a pretty valuable skill in todays league, where switching is very common and to be able to play multiple positions is key, when not a star.
But most of all Smart hustles his tail off. Celtics fans and the announcers often enough call him "Mr winning play". That's not without reason. So while Smart may be a flawed player, when you value shooting over everything else, I think lack of motivation or not caring enough is not a valid criticism in his case.
I honestly feel like Ainge is making a mistake this offseason. Bradley, Crowder and Smart formed the Celtics identity. IT was the main scorer, but those guys were the backbone of the team. Trading one or two of them to create space for Hayward, who is a good player, may backfire when it comes to team chemistry and team identity.