This trade comes as something of a surprise, in the sense that Weiland was expected to be leaned on by Boston in their depleted bullpen. Melancon can do the same, though, has more of a history of big league success, and also costs peanuts -- he is under team control for another five seasons, as he has just over one year of service time.
You might be wondering why the Red Sox couldn't get more for Jed Lowrie, but let's separate what we think Lowrie can do from what he's done for a minute. Lowrie has a bat that could be special at shortstop, but he also has a penchant for injury -- and for taking forever to heal from those injuries -- and isn't going to be confused for a good defensive player anytime soon. His health and fielding -- as well as his platoon splits that make him more valuable against lefties than righties -- make him more a useful complementary piece than a starter. Were he healthy, it would be a different story, but a lack of health has been the excuse for Lowrie for years now. In addition, the Red Sox already have Marco Scutaro at short, and Mike Aviles provides the lefty-mashing Lowrie could have -- and with the added bonus of solid defensive play.
It wouldn't surprise me if Lowrie broke out in Houston at some point, but it would be less surprising if he continued this cycle of progress, injury, regress instead.
Weiland would be a more concerning parting, were they not getting a better version of him in return. Both pitchers feature hard cutters, both induce grounders, and both are capable of missing bats. The Astros will attempt to keep Weiland as a starter, whereas Melancon will move into the bullpen where Weiland would have been, had he remained with the Red Sox.
Melancon has 7.9 strikeouts per nine and a 3.21 ERA in his career, and broke out last season with his 74 inning, 2.79 ERA performance that featured eight strikeouts per nine and a 2.5 K/BB. He will be 27 years old in 2012. It's not clear as of yet if he is going to setup or close, but with his price tag, the Sox can certainly continue to shop for bullpen help.
This is the kind of deal where both sides should be happy. The Red Sox get an inexpensive (and talented) bullpen piece that has had success and is under contract for five more seasons, do so by dealing from a position of depth, and open up a 40-man roster spot, too. The Astros, who are struggling to succeed, get to roll the dice on the oft-injured (but talented) Jed Lowrie, and also attempt to see if Kyle Weiland can continue as a starter. If they are wrong, they can move Weiland to relief, as the Sox likely would have, effectively replacing Melancon's innings in the future.