As a shooting guard, he was steadily improving as a defender. Even spending much of his time now as a point guard his stats are still okay (mostly) but he's just not capable of defending any of the PGs in this league and it puts our entire defensive scheme in jeopardy.
Booker lacks the foot speed and shouldn't be guarding opposing PG's but he is better suited than Tyus, who is just atrocious on defense. I think he's gotten worse as the year has gone on.
I heard some rumblings back when Beal was moved to the bench that Bud wanted to bring Tyus off the bench but he's been mandated to start Tyus since Tyus coming here for the minimum was based on him starting. It'll be interesting to see if he continues to start going forward because I believe Beal is more deserving than Tyus since we aren't utilizing Tyus' PG skills early on anyways.