Lots of cutting and movement.
You can't let the defender sag and stand stationary. You have to make them move back and forth. You have to make them account for backdoor cuts. You have to make that guy have his head on a swivel rather than just sagging into the paint.
Marcus also has to crash the offensive glass with abandon. Since his man is going to be playing off him, he can get a running start on almost every shot attempt. His goal should be to grab at least one offensive board for every stretch he is in the game at a minimum. If he can do that, it will make it harder to play off him because you have to account for him when the shots go up, and its exceptionally tough to run out and box a guy as he's running in to compete for a rebound.
Finally, Marcus has to get something in transition every game. His goal should be one transition bucket, one offensive rebound putback, and one bucket off a cut. That at least keeps the defense honest without him having to make jumpshots. If he can throw in 2 or 3 more offensive rebounds and one more transition opportunity, whether he scores or not, he can be a net positive player without making shots, provided he also provides top notch defense against the other team's best guys.
Unfortunately for Marcus, this means he's going to play 20 of the most high energy minutes you can imagine. On defense, he has to be an absolute stalwart. On offense, he has to be in constant motion, because he has no gravity if he's just stationed out beyond the three point line. Oh, and he has to do all of this while staying within the scheme, both offensively and defensively.