In case you lost track, these teams have been discussing this deal for almost seven months. So it was more than just a rumor. It was practically baseball's version of a stock option. It just took until Thursday for both teams to feel comfortable enough to exercise their options.
You don't have to be a season-ticket holder at Fenway or the BOB to understand this one. Arizona gets to upgrade its out-of-sync offense and save many, many negotiable American dollars. And those were two issues, for this franchise, that weren't going away without some kind of proactive move like this one.
The Red Sox, meanwhile, get a potentially dominating arm who can start, close or serve as a one-man bullpen committee capable of going two, three or even four innings at a time, up to five nights a week. Hillenbrand may have driven in more runs this year than Manny Ramirez. But it won't be offense that stands in the way of this team advancing deep into October.
One scout we surveyed Thursday night had a question we've heard a lot in the last few months: "Why don't they like Shea Hillenbrand? To me, he's got a quick bat, and he's a hell of a player." But it isn't anywhere near as simple as that, of course.
True, the Red Sox had some issues with Hillenbrand's patience and plate discipline. But mostly, he was the perfect trading chip: young, affordable, productive, getting better all the time, and -- for them, at least, expendable. In case you hadn't noticed, this club has bats lined up from Boylston Street to Braintree.
They're second in the major leagues in runs scored. Counting Hillenbrand, they had six different hitters who ranked in the top 10 of at least one of the major offensive categories. So they could score runs with this guy. And they'll score plenty without him.
Bill Mueller, who takes over third base full-time now, is leading the league in doubles. Freddy Sanchez, who comes up from Triple-A Pawtucket to take Hillenbrand's spot on the position-player portion of the roster, was leading the International League in hitting.
So they have players who can do what Hillenbrand did, or a reasonable approximation. But they can use all the B.K. Kims of the world they can get their hands on.
"Without a guy like (Kim)," said one AL scout Thursday, "I don't think they can get where they want to go."
The fact is, though, there is almost nobody like Kim on the planet. For now, with Pedro Martinez on the disabled list, he'll plug into the rotation. And while he may have had only one win in seven starts this year, he'd given up two runs or one in four of them -- and the Diamondbacks had scored exactly eight runs in the 43 innings he was out on the mound.
Down the road, though, he gives the Red Sox lots of options. They still could sign Chuck Finley. Which could push Kim into the bullpen. Or it's possible, one baseball man suggested Thursday, that Pedro could be out longer than advertised, that there might be "a little more there than meets the eye." In which case Kim could stay in the rotation.
"To me," said one scout, "his best role is in the 'pen. I don't think he's as effective going through the order three times."
In time, the Red Sox might find they agree. But that's a question they don't have to answer yet.
The Diamondbacks, on the other hand, had finally convinced themselves they weren't catching anybody in the NL West without more offense. Their third basemen have 17 RBI all season. Hillenbrand had 17 RBI by April 15 (and 38 for the season).
Arizona had scored two runs or fewer 16 times in 53 games. It had been shut out more times (five) than any team in the league. It was scoring 3.3 runs a game on the road -- worst in the major leagues (yep, worse than even the Tigers). The only team in the NL with a lower on-base percentage (.323) was Pittsburgh.
When the manager calls his own offense "a joke," as Bob Brenly did after a 10-2 loss Wednesday in San Francisco, you don't need a translator to understand he'd seen enough.
So even though there had been some private grumbling about Kim's toughness, this was essentially a deal made for two reasons: 1) offense and 2) money.
Because Hillenbrand isn't a fly-ball hitter, Fenway actually hurt him. He hit just 10 of his 33 career homers at home. And his slugging percentage and OPS were more than 100 points higher away from Boston. So he may actually be more productive in Arizona without the Green Monster getting in the way of all his line drives.
But dollar signs were all over this deal, too. The Diamondbacks will save about $1.9 million in salary for the rest of this season. And next year, with Kim potentially due to make $5 million through arbitration, they figure to save at least another $3 million. If that $5 million keeps Curt Schilling in town, they won't have to justify this too hard.
The question, though, is whether they have enough pitching depth to avoid missing Kim now. Randy Johnson and Brandon Webb are on the disabled list. Miguel Batista is serving a 10-game suspension for his role in a brawl. And Matt Mantei had an MRI on his shoulder Thursday after blowing a 13th-inning save Tuesday at Pac Bell.
So on the surface, that looks ominous. But Batista returns Tuesday. Johnson is expected back in a few weeks. Webb isn't believed to be seriously hurt. And Arizona is convinced that over the long haul, pitching won't be an issue.
Which explains why Diamondbacks general manager Joe Garagiola Jr. phoned Red Sox GM Theo Epstein the other day -- and finally made the longest-running trade rumor in America come true.
Jayson Stark is a senior writer for ESPN.com.