Building a formidable bullpen is difficult, tedious and often requires some luck.

There’s so much year-to-year volatility with relievers that an awful bullpen one season can become a strong unit the next, and vice versa. The importance of finding the right mix of arms is especially vital nowadays, however, which makes this offseason interesting for the Boston Red Sox.

Red Sox president of baseball operations Dave Dombrowski essentially needs to build a bullpen from the ground up. He said Tuesday he’ll leave no stone unturned when it comes to fulfilling that task.

Let’s assess Boston’s ‘pen as we turn the page on 2015 and look toward 2016.

2015 at a glance

Complain about the ace-less rotation all you want. The Red Sox’s bullpen was just as much of an issue, though having to pick up the pieces for inconsistent starting pitching admittedly didn’t help matters.

The Red Sox ranked 26th in the majors with a 4.24 bullpen ERA. Boston’s relief corps posted a -1.4 WAR, which was the worst mark in baseball. Only four teams (Red Sox, Atlanta Braves, Detroit Tigers, Oakland Athletics) had bullpens that posted negative WARs.

That really tells you everything you need to now. Koji Uehara and Junichi Tazawa essentially are the team’s only two (usually) reliable relievers, and even they had hiccups in 2015 before their seasons ended prematurely for health reasons.

Alexi Ogando and Tommy Layne led the team with 64 appearances, though Ogando pitched 65 1/3 innings to Layne’s 47 2/3. Tazawa (61 appearances, 58 2/3 innings), Robbie Ross Jr. (54, 60 2/3), Craig Breslow (45, 65), Uehara (43, 40 1/3) and Matt Barnes (32, 43) also had significant workloads.

By the time September rolled around, the Red Sox had several relievers pitching in roles they weren’t accustomed to and the group suffered as a result. One could say it was a mediocre season for the bullpen, but that would imply there were positives, and honestly, those were few and far between.

Whose job(s) to lose?

Uehara and Tazawa are the only locks to return.

Ogando (arbitration-eligible), Ross (arbitration-eligible) and Layne (under contract) could return.

Barnes (assuming they stop bouncing him back and forth between the rotation and bullpen) has the potential to carve out a late-inning role with another step forward. So, too, does Heath Hembree, who throws hard and has an extensive minor league track record that includes closing games.

Beyond that, it’s anyone’s guess. You won’t find an area of the roster with more uncertainty.

Notable prospects (age on Opening Day)

Pat Light, 25

Light hit a brick wall in 2015 upon earning a promotion to Triple-A Pawtucket. The right-hander posted a 5.18 ERA and a 1.73 WHIP in 26 appearances spanning 33 innings with the PawSox.

But the former first-round pick, who just transitioned from a starting role last season, did enough early in the year with Double-A Portland to suggest he could have a future in Boston’s bullpen. Light throws heat and is said to have a closer’s mentality.

Williams Jerez, 23

Jerez was named the Red Sox’s Minor League Pitcher of the Year following a solid season across three levels (Single-A Greenville, High-A Salem and Double-A Portland). The left-hander posted a 2.54 ERA in 88 2/3 innings over 41 appearances between the three destinations.

Jerez, a second-round pick in 2011, is new to pitching — he was drafted as an outfielder and didn’t hit a lick over three minor league seasons — so he’s still raw. But there’s clearly some potential there.

Offseason prediction

The Red Sox will completely overhaul the bullpen via both free agency and trades. There’s no other option.

Now, which types of pitchers will Boston target? Expect a mix, with at least one power arm joining the fray, perhaps even in a ninth-inning role if a top-flight closer can be acquired for a reasonable price.

“It’s a situation where sort of just a mixture of getting the best guys out there at this point to get the job done,” Dombrowski said Tuesday of building the bullpen. “Where that’s going to take us, I don’t know. I’m open to trades, I’m open to free agency, I’m open to hard throwers.

“Now, if Barnes ends up out there, you have a hard thrower. Hembree’s a hard thrower that’s out there, doesn’t strike out a lot of people. Ideally, you want an arm out there that can be a power arm in some role to get a key strikeout at a key time for you.”

Uehara, for the most part, has been excellent as the Red Sox’s closer. But don’t be totally shocked if Dombrowski thinks big and brings in someone like Aroldis Chapman of the Cincinnati Reds or Craig Kimbrel of the San Diego Padres — read: a game-changing closer with a big arm — to completely change the complexion of a unit that currently evokes zero fear whatsoever in the opposition.

Thumbnail photo via Kim Klement/USA TODAY Sports Images