In the deal, the Red Sox also sent cash considerations to the Cardinals, as well as Minor League lefty Corey Littrell, a fifth-round pick by Boston in the 2013 First-Year Player Draft.
The Red Sox gain a considerable amount of contractual control with Kelly and Craig, a right-handed hitter.
"It didn't make any sense to us to trade both Lester and Lackey unless we were getting at least one Major League starting pitcher back," said general manager Ben Cherington. "It just would have made the next several months even more difficult to build to what we want to be. If we were going to do it, we really prioritized getting Major League players, but in particular at least one starting pitcher back."
Kelly, who will move right into Boston's suddenly young starting rotation, has made just three starts this season after missing nearly three months of action with a left hamstring strain sustained while he was running the bases.
Splitting the 2013 season between the rotation and the bullpen, Kelly was a key contributor for a team that won the National League pennant, going 10-5 with a 2.69 ERA in 37 games. In the 15 games he started last season, Kelly was 9-3 with a 2.28 ERA.
In this disjointed season, the righty hasn't been as effective, going 2-2 with a 4.37 ERA over seven outings, all starts.
"In Kelly, we feel like we have a guy who is [coming on] and is a developing, advancing Major League starting pitcher," Cherington said. "He's certainly not a finished product, but really talented. He's someone our scouts have liked for a long time. He's highly athletic, has really good stuff, and we feel someone who can quickly develop into a core part of our rotation. So he was an important addition as we go into the offseason and we wouldn't have done the Lackey deal without getting someone like that back."
A solid offensive performer the last few seasons before struggling thus far in 2014, Craig is signed through '17 and there is a club option for '18. Kelly, who started Game 3 of the 2013 World Series against Boston, isn't eligible for arbitration until after next season. Boston has the 26-year-old under control through '18.
Craig came up big for the Cardinals in October 2011, belting three homers against the Rangers in the World Series. St. Louis won that Fall Classic in seven games. He hit .375 against the Red Sox in last year's World Series, which Boston won in six games.
The trade of Lackey removes nearly all of the veteran presence from a Boston rotation that now has Clay Buchholz, Rubby De La Rosa, Allen Webster, Brandon Workman and Kelly. Anthony Ranaudo, another young gun in Boston's system, will pitch Friday night against the Yankees, a start that was originally supposed to go to Lackey.
"In order to be as good as we can as quickly as we can, we will need to do some work with our starting rotation," said Cherington. "We hope and expect that many of the answers for that can come from guys that are here. But I would expect that we'd be involved in starting pitching this winter."
With their hopes of making this year's postseason no longer realistic, the Red Sox are trying to load up for 2015 and beyond. They got slugger Yoenis Cespedes from Oakland in the deal for Lester.
The Red Sox signed Lackey to a five-year, $82.5 million deal in December 2009 that included a team option at the minimum salary for a sixth season if the righty missed significant time to a pre-existing elbow injury.
That's exactly what happened, as Lackey did not pitch in 2012 following Tommy John surgery.
Just like his friend and former teammate Lester, Lackey goes from last place in the American League East to a club that is in a pennant race. The Cardinals entered Thursday 2 1/2 games back in the NL Central.
There will be quite a reunion next week in St. Louis, when the Red Sox visit the Cardinals for a three-game series from Aug. 5-7.
After battling through inconsistency and injuries in his first three seasons with the Red Sox, Lackey was a strong performer over his past two seasons with the club. In 21 starts this year, Lackey went 11-7 with a 3.60 ERA.
"Lackey made sense for them," said Cherington. "They're trying to win, have a contending team and he's a proven guy and playoff tested, so he makes perfect sense for them. And for them, they had a little bit of a surplus in the outfield, which made Craig available. And since they were getting Lackey back, they could talk about Kelly. The concept ended up being fairly simple in the end ---but sure, we talked about all sorts of stuff."