In recent years, the levels of the minor leagues get harder, and harder to move up. In order to be a successful player in the MLB, you more than likely need to work your way through various minor league levels.. You could start out at extended spring training, and work your way to short season, and then on your way to Low-A, all the way to AAA minor league baseball.
Over the past couple years, I have attended more and more minor league games, mostly at the Low A, and High A levels. I have visited on occasion an AA ball park or two, and once I visited an AAA ball park. I have been around many knowledgeable people who know more about the game than I do. From what I gather, pitchers who pitch at the AAA level, most of them are career minor league pitchers. What I mean is, if a AAA pitcher has two or more years at the AAA level more than likely he has not been successful in the Majors. This is not true for all pitchers; however, I do not want any getting caught up in AAA stats for pitchers.
I have also been rumored to hear, the best pitching comes at the AA level. This is where the real work begins and ends, and AAA is seasoning for hitters. Most or all your best pitchers get their work in at the AA level, and this is where they work out their kinks in order for a couple AAA starts and then onto the Majors. I have recently looked into AAA pitchers stats, and most of them are common of the level of the Majors, however, that is not always great news. If a pitcher has a ERA of 3.50 or higher in AAA, there is a chance that number would increase at the MLB level. If you look at some or most AAA rosters for pitchers, there will be very familiar names there, that is why some pitchers come to contract agreements with teams in what is called, ‘minor league deal’.
Again, this is all opinion and hears say, however, to most a true statement. If you want to look towards the future for your roster for pitching depth, take a look at AA pitcher stats. Pitchers, who can pitch at the AA level, are more successful than pitchers who pitch several years at the AAA level. Keep in mind, these several young pitching starts are approaching or have approached the AA level, and keep an eye on the numbers for future minor league drafts, and possibly future MLB drafts: