It's all in location. He was giving up a lot of HRs because his fastballs were at the top of the zone, centre cut. If he located them down in the zone, and only went up occasionally to alter the batters viewpoint for a strikeout, that HR rate would likely decline. He has the command to locate his pitchers, so that's all that would be needed. Osuna ranged between 96-99, predominatly, in the BP so I could see him live in the 94-96 region as a starter (same with Sanchez). That's more than good enough to minmize contact on the fastball as long he locates appropriately.