Problem is the guys we have coming of IR are probably better than anyone available other than a gross trade overpay.
History shows that a lot of mid-season turnarounds are simply due to players getting healthy or rookies coming out of nowhere. It's not like this team needs a drastic change, small tweaks could have big results, like stabilizing the rotation, which might merely need Arrieta to make an adjustment to be more consistent, Eickhoff a tweak in mechanics to keep his offspeed stuff down in the zone, Pivetta to bottle whatever he had in his last start, and so on - and longer starts mean less pressure on the bullpen.
Just getting back Hunter and Morgan would have a big impact, allowing SD to avoid back to backs (if he doesn't go on IR for a long period), Neris to be used as the closer, and so on.