Even in center field, there really aren’t any impact regulars who fail to hit for at least some power, and barring something unforeseen, I think a lack of thump will keep Rosario from being a true everyday player despite his very interesting, and in some ways very special, skillset. This guy walked in nearly 17% of his Hi-A plate appearances in 2019 as just a teenager, and he has a chance to be a special defender in center because of his speed and athleticism, though he was only a 50 runner last Fall (I’m chalking this up to a long, weird year during which Rosario was traded). Rosario is also very tough to get to swing-and-miss in the zone, but some of that is because he’s very conservatively poking, slapping, and slashing soft contact all over the infield and not really trying to hit for power. Realistically, he’s a low-end regular in center field, or some sort of weird meta-game role player if the plus defense finally materializes.
Basically, they are saying that he'd get promoted because of his speed and his fielding (which hasn't materialized yet).
Sox Prospects has his ETA as "late 2022." Could just be a misprint from Fangraphs.