A prep arm taken in the eighth round in 2013, Keller has been the youngest pitcher in the Cal League for most of the season. His sturdy frame has more durability risk than most on account of a higher-stress motion with a long, low stride that generates out-sized tension and stress on his landing leg. The delivery in general, and arm action in particular, has plenty of length. Coupled with some stiffness in his actions, it leaves the fine command profile behind his advanced control, and it's likely to stay that way. His extension out front helps his average velocity play up, and late-if-limited action with the heater and change make him difficult to square down in the zone. He lacks a put-away pitch and will need to max out the development of his slider in order to hit his no. 4 starter ceiling.