After a major comeback season in 2013 after missing all of 2012 recovering from Tommy John surgery, Lackey was fine in 2014 for the Red Sox. He was well on the way to another one of his better seasons when he was traded. Upon joining the Cardinals, he ran into some poor luck on balls in play, and through 10 starts has a 4.30 ERA. Baseball-Reference has him being worth -0.3 WAR based on runs allowed, but he's actually pitched alright. Fortunately for him, he'll get to the playoffs with the Cards, as they clinched a spot. The Cardinals know all about Lackey's post-season pitching prowess as they faced him in the clincher in the 2013 World Series. Lackey's also promised to honor that option for next season, so the Cardinals aren't just getting a rental and he's shown no signs of deteriorating at 35 years old. He probably easily has one more very good season left in him.
Allen Craig has been dealing with some injury problems this season, especially in his foot. His power evaporated and for a time was one of the worst position players in baseball. With 3 years remaining after this one on his contract, the Cardinals were happy to jettison him onto the willing Red Sox. It was a bizarre looking throw-in for the Sox who didn't really have room for him to play. Since joining the Sox, Craig has been horrendous, batting .128/.234/.191 in 107 at-bats, and cost the Red Sox half-a-win (-0.5 WAR) in the process. As it is, 2014 has just been a lost season for Craig, so the Red Sox are hoping he'll rebound next year, although it's hard to say where he fits into the 2015 lineup.
Joe Kelly was really the prize of the deal, and since coming to the Red Sox, he's been a bit inconsistent. At times, you see the top-of-the-rotation potential clearly obvious. Other times, he starts walking everyone. His 41-to-32 strikeout to walk ratio since joining the Red Sox is not pretty, but as far as results, he's earned 0.3 WAR in his 10 starts. Also, before coming to the Red Sox, Kelly wasn't bad in 7 starts for the Cardinals. If Kelly can improve his command somewhat, he's a fairly valuable pitcher, easily a #3 starter. But until he gets those walks under control and can avoid that "one bad inning" syndrome he's long had, he may only be a guy to run out there every fifth day. If so, you have an extremely good #5 starter. But there's upside here, maybe not quite ace potential, but he's not arbitration eligible until after 2015. So the Red Sox have him under control for a while, making him very easily a long-term asset as long as he stays healthy and can improve his performance slightly.
If in the next three years Allen Craig returns to previous form and Joe Kelly tops out as a #3 or even #4 starter, the Red Sox will "win" in this deal. But the Cardinals got a very good pitcher out of this. It's possible that Craig is just a pumpkin at this point, but they had to take him in order to get Kelly, I'm sure - the way that the Red Sox took Lowell to get Beckett. Yes, I am hoping that this trade turns out much like that now-famous trade with Florida - in which the Red Sox gave up Anibal Sanchez and Hanley Ramirez (the latter of whom the Red Sox would actually miss, the former who only found his groove with the Tigers). Lowell, of course, went on to be very good with the Sox until injuries ended his career during his last season and a half with the team. Beckett would be very good for a few years before going in the mega-deal with the Dodgers.
The only difference here is that the Red Sox only gave up Corey Littrell, not a major prospect, Lackey and cash. It was not a bad trade for the Cardinals, but the Red Sox probably won't miss Lackey's 2015 all that much unless Corey Littrell turns out to be special. If just Craig or Kelly do well, the Sox won't miss Lackey at all.