Fangraphs has him ranked #79 overall as a 50 FV prospect - meaning an average MLB starter. Their #39-100 are all considered 50 FV prospects, so that could be considered a tier, with the difference between any of them being very minor, perhaps negligible (but rankings get clicks).
I think that’s fair. He’s 25…top prospect lists are always going to be dominated at the top by the young, tooled-up guys or the highly-drafted college players who are big-league ready.
Anyway, one thing I really like about the Fangraphs prospect list is that they include a “probability of outcomes” chart for each guy, which provides more context (not sure how they’re calculated, but I’d assume it’s based on age, performance in minors across key metrics like K rate, etc). For Diaz, they have him as about a 25% chance of being a 50/55, 15% chance of being a 60/65 (all-star-ish), and about a 10% chance of being a 70+ (MVP-ish). That feels about right to me, and if you told me right now he’ll be an average MLB starter, I’d take it in a heartbeat.