The trade, which was near completion before Christmas, sent right-handers Mark Melancon and Stolmy Pimentel, infielder Ivan De Jesus and first baseman Jerry Sands to the Pirates in exchange for Hanrahan and infielder Brock Holt.
Manager John Farrell has already spoken to Bailey and Hanrahan.
"It was a great conversation, obviously," Hanrahan said. "He told me they're excited to have me there. ... I'm excited for the opportunity. I wasn't sure what the role was going to be, because there are obviously guys who have closed there before, with great success."
Hanrahan, 31, had a 2.72 ERA in 2012 and a 1.83 ERA the year before -- both times earning All-Star selections -- and he has 76 saves since 2011. His lifetime average of 9.9 strikeouts per nine innings is a positive, too, but his walk rate from last season could be a red flag: that rose, from 2.1 to 5.4 per nine.
The Red Sox said they vetted that matter and that adding a pitcher of Hanrahan's caliber is a rare opportunity.
"His stuff, as I'm sure you've seen, is right at the top of the scale. He's got a fastball in the upper 90s, and when he's right, his slider is as good as anybody's in baseball," assistant general manager Brian O'Halloran said. "I spoke with John earlier, and he has spoken with both Bailey and Hanrahan, so I don't mind sharing with you that John plans to go into the spring with Hanrahan as the closer. He talked to Andrew."
Bailey, 28, missed most of 2012, his first year with the Red Sox, after sustaining a freak injury to his right thumb during Spring Training. He finished with 19 appearances, six saves in nine chances and a 7.04 ERA, and he'll likely get closing opportunities if Hanrahan has pitched on too many consecutive days.
"We see Andrew as playing a very important role in the back end of our bullpen as well," O'Halloran said. "There will be plenty of opportunities for him to help us win games in key situations late in the game, and we know he's very capable both of closing and pitching in other high-leverage situations at the ends of games. It's not too often you get a chance to add a two-time All-Star closer to the mix."
Bailey will, in essence, serve as a backup closer, a role the Red Sox envisioned for Melancon last season. That didn't go as planned. When Bailey got hurt, Melancon was ineffective, and the Red Sox had to put swing reliever Alfredo Aceves in the role.
A closer himself before joining the Red Sox last season, Melancon finished his only year in Boston with a 6.20 ERA, although he let up just one run in September. He told MLB.com from Hawaii on Wednesday that his time with the Red Sox was transformative.
"Obviously, I got off to a rough start," Melancon said. "So, you know, I don't think they treated me unfairly. It's hard for me, because I feel like I didn't produce as well as I should have, and so there's nobody to blame but myself. Obviously, baseball is a game that's built on failure, so you have to understand that, too. It's exciting for me, because I had a great last half of the season last year, and that's kind of who I am and who I anticipate being.
"I learned how to combat [being outside the closer's role], and it took me a little while, understandably, but it made me a better pitcher and a better person. In my mind, there wasn't a [lot of] bad, other than I let down some of the team. Which is never fun. But I think, as a whole, I got a lot better."
For Pittsburgh, the trade could partly have been motivated by the roughly $7 million Hanrahan is set to earn in 2013, his last year before free agency, and Hanrahan was aware that he was likely on the block. He's never been to Fenway Park and only once been to Boston, even though his wife's family comes from Massachusetts (Brockton and Avon).
"I've only seen Fenway on movies," Hanrahan said. "I've never actually been there. I've been to Boston one time for a wedding. That was kind of a quick little trip, in and out of there. I don't know a whole lot about the city or the stadium. I've seen it on TV, but I've never been there."
Just in the number of controllable players, Pirates GM Neal Huntington got a sizable return, but the Sox needed space. Of the four players the Red Sox sent, De Jesus is the only who was not on the 40-man roster.
"Any time you're making a trade or any transaction, you're keeping in mind where your roster sits," O'Halloran said, "but I wouldn't say that that was a big factor in making the trade."
The Red Sox took notice of Hanrahan during an Interleague series in June 2011, when he nailed down consecutive saves while allowing just one hit between the two days.
O'Halloran said that the team is comfortable Hanrahan's walk rate will drop in 2013, and Hanrahan said that he's healthy after dealing with ankle and hamstring issues last season.
"Obviously, we looked at that very closely," O'Halloran said of the walks. "We do think there are some reasons that we saw the uptick in walks. We're going to talk to Joel, and John's already started that process, and [pitching coach] Juan Nieves and [bullpen coach] Gary Tuck and John will get together with Joel and throw out anything that they see and help him with that. It's not something we're concerned about long-term."
Holt, 24 and a ninth-round Draft pick out of Rice University in 2009, made his Major League debut last season in 24 September games at second base. He held his own with a .292/.329/.354 line after he spent the majority of 2012 with Double-A Altoona. There he led the Eastern League with a .322 average and was the fifth-hardest player to strike out.
"He's a very hard-nosed player," O'Halloran said. "We're excited to have him and the energy that he brings to the table. He's got a line-drive stroke, and we think he'll be a good addition to our middle-infield core."
Pimentel, who will be 23 next season, had been in the Red Sox organization since he signed as a non-drafted free agent in July 2006. He started strong in the Minors and found himself on plenty of top-prospect lists, but he hasn't gotten over the hump in Double-A.
Sands' and DeJesus' time in the organization was short. Both 25-year-olds came over as part of the blockbuster trade with the Dodgers last summer. Sands had strong numbers at Triple-A last season, hitting .296 with 26 home runs, 107 RBIs and a .900 OPS. De Jesus played in just eight Major League games with the Red Sox, but hit .304 between Los Angeles' and Boston's Triple-A affiliates.
Only two of the five players the Red Sox acquired in that mega-deal remain their property: pitchers Allen Webster and Rubby De La Rosa.