https://www.whatifsports.com/HBD/Pages/Popups/PlayerRatings.aspx?pid=8197091

I am sitting at 24-21, this player doesn't seem quite ready for the ML, but I have a 74 Pitching coach in AAA and a 96 PC in the ML. I feel like I should promote him to help his development since he is only 21, but I am afraid of the cost of the losses as he grows. Right now, he looks to be maybe a borderline SP5 with the projected ratings that will make him a solid #2 starter. Thoughts?

He's a terrible BL pitcher. You'd be burning service time while he develops, and costing you games, so I can't imagine any scenario where a promotion benefits you.

I don't suspect anyone here can definitively point to a pitcher of theirs and say "This guy was just alright at AAA but then my 91 rated ML coach turned him into a stud."

Also, I'm sure you know this, but young pitchers need innings to develop. That means a prospect in the majors kinda has to be locked into a rotation spot no matter what results he gets. You can't bring him up and fiddle with his playing time.

Agree with both of the above comments. Keep him in AAA.