If Thornburg is still Thornburg, he's as good as any lefty reliever we can get.
He held left-handed batters to a .192 wOBA his last full season, posting absurd reverse splits. Plenty of reason to be skeptical he can do that again, but that mark is upper tier elite — better than Miller, Hand, and Britton — and probably a big reason DD acquired him.
Down the road (let's get some more good days in the books first), healthy Thornburg may affect the case for keeping Johnson around. One thing about having so many lefties in the rotation is that our pen adds 1 and maybe 2 of those guys in a short series. Having one of the RH arms as a weapon against lefties takes some of the regular season pressure off having an "extra" lefty around, though. Plus, you can just add Poyner as a "2nd" lefty whenever someone hits the DL or Velazquez just threw an extended relief outing - Velazquez and Poyner could essentially be one rotating pen spot in that way with Walden, Scott, misc. mixed in as needed until Velazquez's 10 day exodus is up. Johnson may yet have value to this team as the only regularly rostered LHRP, but there's basically no projected need for him in a short series, and Thornburg as a lefty weapon makes us care less if our entire regular season pen has all RH in it at times.
Johnson has always shown solid, if unspectacular, peripherals. He never showed a HR problem in the minors (but with his stuff he may simply be more prone to them with better hitters, running a 1.46 in 49.1 MLB IP so far). His splits would also seem to support that he can be an effective back of staff guy if his exposure to RH is limited, just like Hembree should never be facing a lefty in a hi lev spot. But we are dealing with tiny samples and the splits don't really mean much yet (Lose's point).
I don't think there's any chance we can get him through waivers with how pitching starved a number of teams are. If you commit to giving Velazquez a spot the rest of the way - i.e. he's too good to demote - there's basically no way Johnson isn't a casualty, unless they play dumb DL games all year long. Hopefully I haven't invited bad injury karma juju.