DeRozan is literally a bottom 10 defender in the NBA, even when he gives effort. He has no defensive IQ to speak of. Kuzma is awful too, but at least has length to help mitigate his shortcomings.
Regardless, Kuzma's bad defense was allowed because the Lakers needed his shooting and the gravity he pulled from defenders because teams needed to account for it. DeRozan will bring none of that; teams will sag off of him to double LeBron and AD. In order to maximize his strengths both LeBron and AD would have to stay around the three-point line which is not how you want to use him (nor would they accept that role).