ESPN Insider Keith Law breaks down the Jed Lowrie-Mark Melancon trade here, and I generally agree with his assessment: Lowrie still has upside, although his ability to remain healthy is obviously a question mark.
Melancon, originally acquired by the Astros from the Yankees in the Lance Berkman trade, had a strong season in his first full year in the majors, posting a 2.84 ERA with 20 saves. He was stronger against right-handed batters, allowing a .228 average with a 38/8 strikeout/unintentional walk ratio, versus .243 with a 28/11 strikeout/unintentional walk ratio against lefties. But it's not such a large split that the Red Sox wouldn't be comfortable with him closing games if they move Daniel Bard to the rotation.
Right now, the Boston bullpen lines up something like this, assuming Bard is a starter:
CL Mark Melancon
RHP Alfredo Aceves
RHP Matt Albers
LHP Franklin Morales
RHP Scott Atchison
RHP Bobby Jenks
Other possibilities: Andrew Miller, Felix Doubront, Michael Bowden.
The Red Sox's bullpen was actually fairly strong in 2011 -- well, at least until Bard's late-season collapse and Jonathan Papelbon's blown save in the season finale. Its 3.67 ERA ranked fourth in the AL (behind the Yankees, Angels and Mariners), its strikeout rate ranked third and it allowed the lowest opponents' batting average. The problem, of course, is that Papelbon is gone, Bard may move to the rotation and Aceves may not repeat his strong 2011.
Aceves pitched 93 innings in relief, the most of any reliever in 2011, and just one of two to log 90 innings (Baltimore's Jim Johnson being the other). Will there be a carryover effect? Here's the short list of relievers to throw 90 innings since 2006 and their performance the following season:
Tyler Clippard, 2010 Nationals: 91 IP, 3.07 ERA, 1.21 WHIP. 2011: 88.1 IP, 1.83 ERA, 0.84 WHIP.
Matt Belisle, 2010 Rockies: 92 IP, 2.84 ERA, 1.09 WHIP. 2011: 72 IP, 3.25 ERA, 1.25 WHIP.
Heath Bell, 2007 Padres: 93.2 IP, 2.02 ERA, 0.96 WHIP. 2008: 78 IP, 3.58 ERA, 1.21 WHIP.
Saul Rivera, 2007 Nationals: 93 IP, 3.68 ERA, 1.40 WHIP. 2008: 84 IP, 3.96 ERA, 1.49 WHIP.
Peter Moylan, 2007 Braves: 90 IP, 1.80 ERA, 1.07 WHIP. 2008: Injured (5.2 IP).
Scott Proctor, 2006 Yankees: 102.1 IP, 3.52 ERA, 1.19 WHIP. 2007: 86.1 IP, 3.65 ERA, 1.46 WHIP.
Salomon Torrees, 2006 Pirates: 93.1 IP, 3.28 ERA, 1.46 WHIP. 2007: 52.2 IP, 5.47 ERA, 1.41 WHIP.
Jon Rauch, 2006 Nationals: 91.1 IP, 3.35 ERA, 1.25 WHIP. 2007: 87.1 IP, 3.88 ERA, 1.10 WHIP.
Geoff Geary, 2006 Phillies: 91.1 IP, 2.96 ERA, 1.35 WHIP. 2007: 67.1 IP, 4.41 ERA, 1.44 WHIP.
Certainly, Clippard and Belisle are best-case scenarios here. I don't think Aceves' season was fluke -- he's now pitched 240 innings in the major leagues with a career ERA of 2.93 and opponents' batting line of .213/.287/.341. But when he pitched 84 innings with the Yankees in 2009 (with all but one appearance coming in relief), he spent most of 2010 on the disabled list. With Bobby Jenks' health an issue after back surgery this month, I suspect the Red Sox will still look for at least one more arm to bolster it bullpen depth.