For a large portion of the 2013 season, the Yankees' bullpen was considered their biggest strength. However, with the departure of Mariano Rivera, as well as the (inevitable) departures of Joba Chamberlain and Boone Logan, the Yankees should look to strengthen their bullpen this winter, even if it may not be the team's number one priority.
A lot of talk this winter, and even more so when we get into Spring Training, will be, barring a free agent signing/trade of a closer, David Robertson's transition from pitching primarily in the eighth inning to pitching in the ninth inning. Personally, I don't see this is as much of a problem. Since the start of 2011, Robertson has been nails, pitching to a 1.91 ERA (220 ERA+) and 2.31 FIP in 205 games and 193.2 innings. The ninth inning shouldn't, in theory, be a problem. It's the the few innings before that that could get a little dicey.
Shawn Kelley, Logan, Preston Claiborne, and Joba handled innings six-eight, with Adam Warren handling long-man duties, for the most part, in 2013. At least for now, Kelley figures to move up to the eighth inning, while Claiborne, Dellin Betances, and Cesar Cabral handle the innings before that with Warren, barring a move to the starting rotation, sticking in his long-man role. That's a little shaky. Claiborne got clobbered the final three and a half months of the year, Betances is still very unproven despite a resurgence out of the bullpen in Triple-A, and Cabral is probably just a LOOGY moving forward. Thankfully, there's potential for some help in the relief free agent market.
Looking at the list available free agent relievers, there are seven who could be targets, including Matt Albers, Luis Ayala, Jason Frasor, Matt Lindstrom, Nick Masset, Joe Smith, and Edward Mujica, who has 37 saves this year but lost his closer's job to Trevor Rosenthal late in the year. There are also options at closer, including Grant Balfour, Joaquin Benoit, Fernando Rodney (gross), and Joe Nathan. At the same time, all of those options figure to come at a hefty price, and the Yankees are covered in the ninth with David Robertson, as noted earlier, anyway.
There are also in-house options the Yankees could turn to if they decide to not dip into free agency. Mark Montgomery still has some prospect shine, but he struggled in 2013 with shoulder troubles, and when he was healthy he issued more than his fair share of walks (5.6 BB/9). There's also Tom Kahnle and Dan Burawa, who have very good strikeout rates (11.1 K/9 and 9.0 K/9, respectively), but have trouble with walks (6.8 BB/9 and 5.7 BB/9, respectively) and have yet to pitch in Triple-A. I'd have to think these are more last resort options if anything, though they could get a chance sometime during 2014 if things go wrong.
A reason for the Yankees' inability to steal a wild card spot late in the year was due to their bullpen meltdown in September. In the regular season's final month, the Yankees' relief corps pitched to an American League worst 5.62 ERA and 4.78 FIP. Guys like Robertson, Kelley, and Logan all got injured/ran out of gas for pitching in countless high-leverage situations during the year, thus Claiborne and Joba, to go along with various veteran minor league scraps, got further exposed. If the Yankees don't go out and make a bullpen upgrade or two in the off-season, we could see more of the same in 2014.