According to Nick Cafardo of the Boston Globe, the Red Sox have indeed exercised that last option for Mark Melancon, and sent him to Pawtucket in order to give him the chance to fix what's wrong with him.
To replace Melancon on the roster, the Red Sox have brought up right-hander Junichi Tazawa. Tazawa missed most of the 2011 season recovering from Tommy John surgery, but after working his way through the minors, finished the year with Boston. He threw all of three innings, and he's now further from that surgery, so it's tough to gauge just how representative that fleeting moment is in regards to projecting his 2012. He was an actual prospect before the elbow woes, though, and after a rough start, he eventually mowed down minor-league competition last year. (11.9 strikeouts per nine, 6.3 K/BB at Pawtucket in 14 innings, after 10.6, 3.9, and 23 for Double-A Portland.)
Melancon's demotion comes on the heels of his meltdown in Tuesday night's game against the Rangers, in which he was unable to hit many of Jarrod Saltalamacchia's targets, and gave up six runs, three homers, and failed to retire a single hitter. Pitching in games that won't affect Boston's standings is the best thing for Melancon at this point, who is (a and was expected to be) a valuable contributor out of the Red Sox bullpen this year.
The pen is likely less productive with Tazawa than Melancon, but since Tazawa isn't replacing the useful version of Melancon, this move represents a short-term step up over what they've been getting. And, since Tazawa still has the ability to be optioned to Pawtucket, the Red Sox won't lose him if or when he needs to be sent back down.