I can't say I'm a fan of E-Jax in the starting rotation. He's too prone to concentration lapses throughout the course of a start and just starts throwing too many balls and putting guys on, then overcompensates by hanging meatballs over the plate for 3-run homers. There's something we usually do with pitchers who have phenomenal stuff but cant cut it as starters. Move them to the bullpen! How much more sample size do we need to prove Jackson isn't really cut out as a starter.
My suggestion is to cut Glover (I know, calling for Glover's job, real original), move Jackson to his role, and bring up Niemann to start. Don't make Jackson a long reliever like Hammel, or a multi-inning guy like Howell. Just get him used to coming in for one inning, throwing 96+ heat with a high 80's slider and getting guys out to get his confidence up. He has the potential to be a killer closer, and Percival is the kind of guy who could help teach him the tricks of the trade for the 'pen. Niemann couldn't possibly be anymore of a rollercoaster ride during starts than Jackson is, and it's time to give the #4 draft pick an extended look in the major leagues.