There have been reports indicating that Rozier might have gone in the late teens, so from that perspective it's not that much of a reach.
It does certainly feel like a reach given that the consensus seemed to be that Rozier was at the least one of a half dozen or more similarly promising players, and he makes very little sense for the Celtics from a roster fit / development opportunity standpoint.
At the time, I felt that once Ainge failed to move up to a spot in the draft where he could take somebody he really wanted, he decided to take a guy at #16 with a skillset he felt comfortable projecting to the NBA. Basically, Rozier felt like yet another "safe" pick from Ainge, even though it was also a reach.
Still feel that way now. Rozier looks decent in Summer League, but he's old for a rookie and I'm not sure how much upside he really has. Chances are Ainge used the #16 pick on CJ Watson 2.0.
I love how Smart (20) was spoken about like he was young for a rookie, yet Rozier (21) is referred to as being old for a rookie. Like the difference of one year in age is going to determine whether a player has potential or not. There have been a LOT of great NBA players who came in to the NBA at 21-22 years old. Dwyane Wade, Stephen Curry, Michael Jordan and LaMarcus Aldridge - just to name a few. Could build a 20-long list without much trouble.
Anyways, I think it's easy to see why Danny drafted Rozier:
a) He can bring it on both ends of the court, at either guard position
b) He's got excellent physical tools (great length, elite quickness, good strength)
c) Workhorse with an excellent motor - plays with 110% effort and hustle 100% of the time
d) Has a 'never back down' attitude, and shows a genuine desire to be great
e) Solid skill base to develop from
Remember that DA and BS seem to be just as big on personality as they are on talent. They love guys who play with a chip on their shoulder and who simply out-work and out-hustle their opponents every time down the court (Bass, Crowder, Bradley, Smart, Jerebko, etc). They also love guys who have super high basketball IQ (Sully, Olynyk, Hunter, etc).
None of these draft selections are surprising if you look at it that way.