Good lord, why? He's another left-handed bat (you want a left-hand corner, the Adam Lind era could begin any time....), would cost us something in prospects, would push us even closer to the larger salary penalty, and we have plenty of infielders, especially with Brandon Phillips moving to AAA today.
What we need is an insurance starter. Without the injury to Vasquez, Swihart would have been our trade bait; now, who knows? Still, an inning-eater would allow us to use Velazquez in a role suited for him, and we'll have to live with Johnson as a starter till Wright is back or Pom figures it out.
We really don't have a critical need for anything else out there, especially if Braiser and Thornburg are what they appear to be.
Why? What "innings-eater" improves this team's chances of making or going deep into the post-season? Put differently, what starting pitcher improves on Velazquez and/or Johnson? I could possibly get on board with the idea to go get a starter who would be penciled into the post-season rotation (ahead of 3 of Pom/Porcello/Price/EdRo?), which would require significant trade assets/salary we may not have. But Velazquez and Johnson have both been serviceable as either starters or long relievers. A better use of resources would be to get an 8th inning type arm. I've been impressed with Brasier, and I hope he (or Thornburg, like you also suggest) can be that addition. Kelly righting the ship is important, too, in that regard. But neither Velazquez or Johnson are a late-inning guy. Moving them into the 5th starter slot doesn't significantly alter the makeup of the playoff staff where you're using 4 starters and relying on your back-end relievers. Really, losing 2 of EdRo/Pom/Wright weakens the bullpen more than the starters, because it means you aren't adding one of the starters to the bullpen in the playoffs. I'll also add that I think losing Rodriguez hurts more than the others, since he's been the 2nd best starter thus far. If he's not back to form by the playoffs, they could be in a tough spot absent other guys stepping up. But, again, an "innings-eater" doesn't solve that problem.
Johnson overall: 24 G, 44.2 IP, 7.46 K/9, 2.62 BB/9, .343 BABIP, 4.23 ERA, 3.81 FIP, 4.33 xFIP
As starter: 3 G, 14.2 IP, 5.52 K/9, 3.07 BB/9, .313 BABIP, 2.45 ERA, 4.71 FIP, 5.05 xFIP
As reliever: 21 G, 30 IP, 8.40 K/9, 2.40 BB/9, .359 BABIP, 5.10 ERA, 3.37 FIP, 3.98 xFIP
Velazquez overall: 28 G, 50.2 IP, 5.86 K/9, 2.49 BB/9, .323 BABIP, 2.66 ERA, 4.13 FIP, 4.45 xFIP
As starter: 3 G, 13.2 IP, 7.90 K/9, 2.63 BB/9, .263 BABIP, 2.63 ERA, 5.11 FIP, 4.53 xFIP
As reliever: 25 G, 37 IP, 5.11 K/9, 2.43 BB/9, .341 BABIP, 2.68 ERA, 3.76 FIP, 4.43 xFIP