Reading between the lines of this article on the Diamondbacks' roster, it looks like Arizona won't be able to keep Brett Lorin unless they can make a trade with the Pirates to keep him. The D-Backs' starting rotation is accounted for, and so is their bullpen. They've got what appears to be a seven-man 'pen in J.J. Putz, David Hernandez, Bryan Shaw, Brad Ziegler, Takashi Saito, and lefties Craig Breslow and Joe Paterson. After that, they also have to deal with lefty Mike Zagurski, who they risk losing on waivers if they try to send to the minors. (One possibility might be for the Diamondbacks to go with an eight-man bullpen until they need a fifth starter in mid-April, but even then, they might take Zagurski before they took Lorin.) There's also lefty Patrick Corbin, who has pitched well in Spring Training, but will probably be sent to the minors.
The end result is that Lorin isn't even mentioned in the article. This one, from a week ago, does mention him:
The right-hander has thrown well this spring, but with seemingly no spots open in the bullpen we haven't really talked much about him. It will be interesting to see at the end of spring if the D-backs try to work out a deal with the Pirates to keep Lorin and send him to the Minors.
That's a tactic the Pirates themselves have used before -- they picked Evan Meek in the Rule 5 draft, then bought him from the Rays when it turned out they couldn't keep him on the roster the whole year. I assume the Rays got practically nothing out of it (though I don't think the exact terms of the deal were ever revealed), and I was never sure why they did that. I know they had a ton of young arms, but an asset is an asset. I doubt the Pirates will be so generous when it comes to Lorin. Not that losing him would be a huge deal, but I'd like to see what he can do at the higher levels of the minors.
For whatever it's worth, Lorin has pitched seven innings so far this spring and has allowed one homer (an inside-the-park job, not even one that cleared the fence) and no walks, while striking out five.