I remember we tried playing Smart against KD last year. That didn't work because KD could just shoot over him. My guess is, Smart matches up better against LeBron because they're both more physical players, I think Marcus can handle LeBron better than he handled KD.
I think Jaylen isn't always locked in defensively, and even when he is, he's not assertive enough, strong enough, to really get in a guy's way and block him off. He often ends up fading away at the last second and the offensive player has a good look at the rim.
If we wanna have a better chance at winning the game, we play Smart on LBJ. If we want Jaylen to improve and develop, we play Jaylen on LBJ