Bannister was passed over in the draft last year despite going 7-5 with a 3.98 ERA in his junior campaign at Arizona, and after having a dominating summer in the Cape Cod League (4-1, 2.37 ERA) the year before.
Many players have benefited from the new Arizona coaching staff coming in this year, but I don't know that anyone has benefited more than Bannister. Over the course of his senior year, Bannister has turned into a dominating pitcher, and has also picked up about 3-4 MPH of velocity on his pitches, now running fastballs up to the plate at 87 or 88 MPH.
When he first came to Tucson from Peoria's Liberty HS, he didn't get many opportunities, appearing in just three games his freshman season. He got more opportunities in his sophomore year, but was also constantly messing with his arm slot and overall mechanics, probably causing him to not develop in the way he should have.
Now under Dave Lawn, Bannister's delivery is smooth and easy, and is highly repeatable. He has transformed his body in a way where most people probably don't even recognize him. And he's also been able to add a fourth pitch to his repertoire.
This allowed him to post a team-best 2.62 ERA and 11-2 record this year, also tossing three complete games along the way. He's also up to 93 strikeouts in 126 2/3 innings. Compare that to the 59 whiffs in 83 2/3 innings last year.
All in all, the Mariners organization will get a still-improving Bannister, and one that could help them in the short-term as well as the long-term. This is the first time the Mariners have drafted an Arizona player since Brad Boyer in 2005.
Stats via thebaseballcube.com