I don't think anyone has regressed under Watson's tenure.
Booker's the main individual player. Watson took over at roughly the All-Star Break last season, so it's pretty easy to compare statistics from the last half-season of Hornacek to the year under Watson.
Booker's shooting numbers:
2015-16 pre-AllStar (Hornacek): .450 overall, .403 3FG, 1.31 points per shot
2015-16 post-AllStar (Watson): .401, .287, 1.13
2016-17 (Watson): .401, .323, 1.10
Knight's problem, as we have all seen, is that he is in love with the shot just a few inches inside the three-point line, which is usually called the worst shot in all of basketball. The way to see that is to look at his 2FG%:
2015-16 pre: 2FG% .475
2015-16 post: .377
2016-17: .415
Tucker can't make three pointers anymore:
2015-16 pre: .346 3FG
2015-16 post: .304
2016-17: .325
The team's assist/turnover ratio, which was already bad, has gotten worse:
2015-16 pre: 1.26
2015-16 post: 1.23
2016-17: 1.18
The team's increasingly lazy defense is captured in the greater number of fouls:
2015-16 pre: 22.2 fouls per game
2015-16 post: 23.6
2016-17: 25.5
Len got a big bump when Watson took over, but has given almost all of it back:
2015-16 pre: 19.5 mpg, 6.2 rpg
2015-16 post: 30.6, 10.3
2017: 22.3, 7.3
And it sure looks as though team defense has gotten worse, although I'm not going to dig into the numbers to prove it.
So basically, since Watson has taken over,
* shot selection has gotten worse
* ball movement has gotten worse
* accountability on defense has gotten worse
* the team's least disciplined players (Knight and Tucker) are taking more bad shots
* Len's apparent progress has reversed itself
Gee, what's not to like?