The Red Sox improved their bullpen significantly by adding Craig Kimbrel as their closer earlier this month. The move not only gave them Kimbrel, but also moved Koji Uehara back to a setup role and made it that much more likely that Junichi Tazawa would avoid overuse, and therefore remain effective.
Even with those three and a seemingly rejuvenated Robbie Ross, though, the Red Sox pen could still use some help, so it's no surprise to see they are still "actively" searching for relievers, according to ESPN's Jerry Crasnick.
They are far from alone in this, with Crasnick also mentioning the Diamondbacks, Braves, Tigers, Astros, Royals, Dodgers, Mets, A's, Pirates, Blue Jays, and Nationals in this way. The reason to bring it all up now, though, is that the expectation, per Crasnick, is that the reliever market is going to get moving this week before the winter meetings can begin next week. The holdup to this point has been the trade markets for Aroldis Chapman and Mark Melancon, but even with those still in play, the time to start signing seems to be right about now.
The Red Sox don't need another closer, but they have room for another setup arm in order to protect themselves against Koji's age and health and Tazawa's effectiveness over a full season. They have relief options in the minors they could turn to, but none of them are reliable at this stage -- some of them might never be reliable -- so another arm they could count on would make a huge difference. Remember, as poor as the rotation was in 2015, the bullpen was worse. Adding Kimbrel helps, but there is more work to be done to make sure the pen isn't a problem once again.
Someone like Ryan Madson would be a huge get for the Sox. Darren O'Day is still out there if the Sox want to invest in what could be a post-Koji future in 2017 and beyond, but that seems less realistic with Kimbrel now aboard. There are other, less appealing options, but they are still beyond what the Sox have available to them in-house at the moment, so expect them to stay interested in the market even if the likes of O'Day and Madson are unavailable to them.