Wow. It didn't seem likely that Byrd would go unclaimed for the Pirates to strike a deal. I'm surprised the Reds didn't claim him, and if I were a Reds fan, I'd be baffled that they let the Pirates do this.
Anyway, for what it's worth (and for the few dozen games remaining in the season, it may not be much, but it's an important few dozen games), this is a nice upgrade in the outfield. Byrd is having a much better season than Justin Morneau. It's a career year and he's 36, so he doesn't project to continue hitting at a .285/.330/.518 pace, but he has a decent history of hitting for average, and he's also a solid defensive outfielder. Hopefully the Pirates didn't feel like their hand (no pun intended) was forced because of something they know about Starling Marte's injury, but they could have used an outfield upgrade either way.
With a sample size as small as the one the Pirates are getting, it's obviously possible Byrd won't help much, but these are high-leverage games. Being forced into a one-game playoff cuts the Pirates' postseason hopes in half from the start, so it's important that they try to win their division. And then, of course, there's the postseason itself. It was frustrating that the Pirates didn't acquire someone like Byrd at the July deadline, but they're making up for it now.
Buck hits for low averages, but he'll take a walk and has very good power for a catcher. He lost his starting job when the Mets promoted Travis d'Arnaud a week or so ago. I assume he'll back up Russell Martin.
Buck is making $6 million this year. Byrd is a bargain at $700,000, so it makes sense that the Pirates gave up a real prospect here. Assuming the PTBNL isn't anyone important, this is a fair trade. Herrera is promising, and 19-year-olds who can hold their own in full-season ball don't grow on trees, but he's a long way from the majors, and he's never been considered an elite prospect. He was also pretty far down the Pirates' list, which is a testament to their depth. This is a risk, but given the position in which the Pirates find themselves, it's a good one.