The kid was making his major league debut after pitching a total of 3.1 IP in a game in the last year and a half. it is WAY too early to pronounce him as anything. BUT one thing you can do is look at the raw stuff and see that it is in the top 1% of baseball. Even if a bit comes off of velocity as a starter, you can see through a guy like Chris Sale that you can work in the mid 90's with good off speed stuff and a deceptive motion and release. Just for the little bit that he pitched the other day, you can see how his stuff could play in the starting rotation, it is a matter of how healthy they can keep him and if he can build up to 110 pitches every 5 days for a whole season. If anyone can get him there, we saw the Sox do it midstream with both Sale and Quintana.
Another thing worth noting is that the Sox are notorious for simplifying a pitchers offerings during their early appearances with the team, and then letting them add back to them as they get comfortable with the initial offerings. With a guy like Crochet, even if they make him a basic fastball/slider pitcher out of the pen to start with this year, and even into when he is a starter again next year, they will eventually add something like change up or a slower fastball with more movement (4 seam/cutter, etc)
Even a guy like Cease isn't a finished product at this point. He has no fastball control, so they aren't going to let him expand his offerings until he has his basics down. This is how the Sox operate.