I get being low on Grant as a prospect- he's really undersized and not particularly athletic, and those are two things that usually don't fly in the NBA. From what I've read, front offices were all over the place with him, with some having him as a second rounder due to those concerns. That's where Radsoxfan is, and it's worthwhile to question whether having a guy in Tucker, who is really a one-of-a-kind type player as a median-ish outcome mold, is wise even if one thinks that Grant is really similar at the same age. But for the people who are low on him because of how he has played recently, I just think people sort of forget how development works for 90% of NBA players-and 98% of non-lottery picks. Rozier's a guy who I think has a fairly typical positive outcome development curve- he was absolutely terrible his first two years, then started to show out in his third year, and continued to have ups and downs as the game slowed down for him and he became a decent NBA player.
Especially for a guy like Grant, who basically has had to change his entire game because how he played in college was never going to work in the NBA, going from a post player who basically ran the offense to a (hopefully) perimeter player who sets screens and sits in the corner while focusing on defense, there was always going to be a very steep learning curve. My belief and hope is that if Ainge believed in him enough to draft him, then as long as he's putting in the effort and showing progress, the organization will give him sufficient time to learn how to play this new game.
My only really quibble with anything Radsox has said is that I would be very surprised if PJ Tucker didn't have an 80 minute stretch where he looked overwhelmed guarding wings, even when he was older than Grant is now and playing for BC Donetsk, or Aris BC, or Bnai Herzliya Basket, or Brose Bamberg. If you take, what's essentially a 3 game sample of any player's career, you can find some really bad tape. Especially when they're playing with new teammates, and especially after a very weird year.
The conditioning issue is definitely a yellow flag for me. I agree with Cesar above. If Grant wants to look to Tucker as a role model, he should understand that at this stage, his career is on a knife's edge and if he isn't putting everything into it, he could end up spending the next 5 years bouncing around Europe too, and he might make it back. I get the sense that in the NBA there's a pretty strong ethos of "bust your ass, because there's always someone coming up to take your job", and if he doesn't internalize that, the draft and free agency can make us forget about him pretty quickly.