Rust is doing a lot of gliding and watching. He's not moving his feet and he's not using any remaining speed he may have to his advantage. Thus, when those perfect tippy tappy plays aren't foregone conclusions for him, he struggles. He needs to "work" to get out of this rut. That starts with solid defense, moving your feet, going the extra mile in the corners, etc. There's just a lot missing from him game at the moment and right now, he can only blame himself because Sullivan is giving him the opportunity to get out of it by putting him with Malkin. If it was Kapanen and he turned in the exact same performance, Sullivan would have yanked him off that line well before now.
The good news is that Rust is still a good player and history suggests that he WILL snap out of it. Once he does, it will be a big help for the entire team.