not sure where you're seeing that he's getting hit hard well above MLB average. during the period that i mentioned (june 1st onwards) his HH rate is 42.5% (league average 41%) and he's exactly league average in barrel rate (8.6%). the hr/9 is down but his hr/fb% in that period is 9%, a bit below league average but nothing crazy or unsustainable (the marlins as a whole this season are at 9.5%). FIP isn't my favourite metric but it still does a better job of predicting future ERA then past ERA does.
the lack of whiffs is obviously concerning but since the stuff is essentially the same & his command has been coming back, you might be able to get that back with some usage adjustments:
against righties in particular, the marlins have been pushing the 4 seam usage (especially when ahead in the count) and it just has not worked at all (4.5% swinging strike rate, real bad damage metrics). backing off that for more breaking ball or offspeed seems like something an acquiring team would try.
against lefties, he used to throw nearly 40% changeup when behind in the count & had great results limiting hard contact. this season, the usage is down 20% despite the changeup still having a 45% whiff rate & below average hard hit rate in that situation
guys with top shelf stuff & this much team control just do not become available very often. we've been trying to sign top of the rotation guys for the past few offseasons without any luck. now they have a chance to acquire someone who has that ceiling, has already had his major elbow surgery & is going at a slight discount, for some prospects that MIGHT be big leaguers one day. even if they technically lose the trade from a value perspective, taking that kind of risk now is exactly what the team should be doing if they're trying to win