I disagree. That upside, as a starting pitcher, is largely dependent on Brown developing a serviceable third pitch. That isn't going to happen in-season while starting for the Major League team.
He either needs to go to Iowa and work on things to give himself a chance to be a successful starter, or move to the bullpen where his current repertoire is more than enough to get by.
Continuing to run him out as a starter isn't doing anyone any favors, most of all the Cubs bullpen.