So as someone that has watched every THJ game the past few years, he sucks. Can't (or wont) pass, cant drive, doesn't play any type of defense. 3-5 nights this season he will blow up and hang 40+, maybe even a 50 piece. But the other nights you will absolutely hate him with a red hot passion. As a bonus, his dad will bitch and moan about Detroit's coaches and/or players on social media and to any press that will listen. He seriously tried to say the Mavs were better without Luka.
The Mavs have been attempting to trade him for years and nobody ever bit. You might flip him for some seconds at the deadline as an expiring. No contender will want him. The Mavs benched him in the playoffs because he is so bad.
Giving up Grimes? Look, I don't think he is that good. But there was zero reason to trade him. Dallas had to move THJ so they could keep DJJ. Grimes will most likely have more value to a contender at the deadline than THJ will. A guy who plays decent defense with a decent three ball on a small contract is very valuable to teams entering playoff runs. Huge mistake to move him in this deal.