Taking his immortality out of the equation, assuming this is anime continuity and saying this is initial or Halo Zamasu, he gets stomped by initial Jiren. SSB Goku was able to overpower him in a beam struggle, so the resulting Zamasu would be weaker than the KKx20 SSB Goku Jiren stomped.
If it's Mutated or Giant Zamasu, he stomps. I would say either version's base level ought to be far weaker than the Genki-Dama that initial Jiren had some struggle with and I'd say having much more than a 2-3x gap between Jiren's initial and full power/limits broken levels would be a stretch when considering Toppo implied Galu's power in all 3 instances of Ultra Instinct weren't that far apart from each other.