He's a "grappler", yes... but not a very good one. On the feet he has passable boxing fundamentals but is by no means a particularly good striker and doesn't have much power in his hands for a Heavyweight. He poses a theoretical takedown & submission threat, but only against fairly low-level guys. Tanner Boser was undersized at HW and had awful defensive grappling, but Nascimento struggled to do anything other than sloppily drag him down and lay on him while doing nothing with the position in between occasionally controlling him against the cage. He never even got close to actually threatening with his ground control and everything he did was labored. He barely escaped with a narrow and questionable Split in that fight... the dude's entire resume to get ranked is sus.
Lewis has historically struggled with grapplers, sure, but better ones than Nascimento. The Spivak loss was a bad look, but the fact that he survived underneath Almeida (essentially a faster, stronger, more athletic, and more talented version of Nascimento in many regards) for five rounds straight when no one else had up to that point -- on short notice, no less! -- sort of vindicates him in my eyes. Hell, he even managed to gain separation, dominant positions, and land some shots to threaten Almeida here and there.
Blaydes also failed outright at taking Derrick down, which a lot of people forget.
I would give Nascimento a small chance at pulling something off... probably by catching an RNC or some other sub in a scramble (because I'm not sure his cardio is up to the task of grinding out five rounds with his mediocre wrestling). But after watching tape on both? Give me Lewis by KO/TKO, probably inside the first ten minutes.