So, I share your concerns to some degree. I am really not sure how he will be able to guard typical NBA guards, but the thing is that the alternative (Young) is actually quite a bit worse defensively.
As you pointed out, the big knock on Booker was speed and athleticism. Same goes for Warren. I tend to think that "athleticism" definition has really gone to hell over the last few years. Doncic is not super-quick or explosive, but he has a good balance, understands the angles really well, so I am not overly concerned with his "lack of athleticism." Again, Young is not really explosive or quick either.
I will tell you that he is an outstanding passer, a very good shooter, and an incredibly smart player. One hell of a competitor too. The games I've seen, he was better than Dragic right now. I can't remember who Slovenia played, but I watched two games where he just carved their defense like a Thanksgiving turkey. It was like watching Nash play back in the day. I hate cliches, but I really think he would make every single player on the roster better. Booker's job would be much easier, Warren would get plenty of easy layups, Jackson would get plenty of easy dunks, and Bender would get a ton of open three pointers. Let me do some digging and see if I can find highlights at least.