Over the last few years, the Cubs have made bullpen construction look easy. 

Relievers are notoriously volatile and it’s nearly impossible to predict what you’ll get from one year to the next. 

The best closer in the game last year (Edwin Diaz) is just a few years removed from a season in which he put up a 5.59 ERA. Meanwhile another elite closer (Josh Hader) posted an ERA over 5 last season after a Hall of Fame caliber start to his career.

Yet the Cubs front office has hit on arm after arm in recent offseasons, adding under-the-radar pieces that emerge as key parts of the bullpen. Jeremy Jeffress, Ryan Tepera, Andrew Chafin, David Robertson, Chris Martin, Mychal Givens — the list goes on and on.

So when Jed Hoyer’s front office signed a pair of veterans — Brad Boxberger and Michael Fulmer — and claimed another off waivers (Julian Merryweather), the expectations were high that these moves would pay off.

Combined with some exciting homegrown arms (Brandon Hughes, Keegan Thompson, Adbert Alzolay, etc.), the Cubs seemed primed for another season of successful bullpen work.

Only it hasn’t played out that way. 

Boxberger is currently on the IL with a forearm strain and had a 5.52 ERA before hitting the shelf. Fulmer started the season off on an impressive note but has since fallen on tough times and carries a 7.20 ERA while converting 1 of his 3 save chances.

Hughes has had two separate stints on the IL and Thompson was optioned back to the minors late last week.

Entering play Wednesday, the Cubs rank 22nd in the league in bullpen ERA (4.39) and that ranking falls to 26th in the month of May (4.95 ERA).

“I feel like I’ve put [manager David Ross] in a tough spot to a certain extent,” Hoyer said. “That’s an area we’ve had so much success with, whether it was Chafin or Tepera or Martin or David Robertson. We’ve candidly done a really good job of finding relievers that could come in and throw high-leverage innings at a relatively low cost on 1-year deals.

“We’ve been building bullpens that way for a while. This year, that hasn’t worked yet and that’s on me. That’s put Rossy in a tough spot. That hasn’t lined up the way we expected it to.”

Because of the inconsistency, the Cubs have been forced to make a variety of roster moves to tinker with the bullpen mix. Rookie Jeremiah Estrada was the first guy out of the bullpen Tuesday night, called upon to escape a bases-loaded, nobody out jam with NL home run and RBI leader Pete Alonso at the plate in the 6th inning.

Estrada showed his mettle, allowing only 1 run and helping preserve a much-needed victory for his team. 

But that was also Estrada’s first opportunity in a high-leverage situation and he received his chance because Ross doesn’t have a lot of trustworthy options right now. It’s nearly Memorial Day and the Cubs don’t have a clear closer nor defined roles in the bullpen.

“To this point, nobody’s grabbed that role and took it to be the guy that’s pitching the 9th inning,” Hoyer said. “As a result, we’ve been matching up more. There’s been times we’ve had the wrong matchup at the wrong time and it’s burned us. There’s times that we’ve had the absolute right matchup on paper.

“We need that area of the team to stabilize. We’ll continue to work through different matchups and work through potentially adding different guys. We have to get that area right.”

So how do the Cubs stabilize the bullpen?

It helps when the starting pitcher goes 8 strong innings, as was the case Wednesday night with Marcus Stroman. That afforded Ross the luxury of only needing one arm out of the bullpen as he called on Mark Leiter Jr. to get the save.

But the Cubs can’t ask for 8 innings every night from a rotation that has already been a major strength of the team.

One answer for the bullpen could be turning to the young arms coming up through the farm system.

“I think we have a lot of young guys that are gonna be really quality relievers in the big leagues,” Hoyer said. “But it takes time to get there. Not only time to get through the minor leagues but also time to maybe take some lumps in the big leagues until you figure it out, so we’re going to have to live with some of that.”

Estrada is one of those power arms, as he posted a 0.96 ERA in 8 appearances with Triple-A Iowa before his promotion to Chicago. 

Behind him are young relievers like southpaw Bailey Horn and righty Cam Sanders. The Cubs are also moving pitching prospect Daniel Palencia to the bullpen to maximize his 102-mph fastball and he could be the first of several such moves.

“It’s nice to have those kinds of arms,” Hoyer said. “We’ll be transitioning those guys into the bullpen in the weeks and months to come.”

The Cubs bullpen could receive a boost in the near future with the return of Codi Heuer, who has missed the last year-plus after undergoing Tommy John surgery last March. He is eligible to come off the 60-day IL on Monday but his rehab outings in Iowa have been inconsistent. 

He had a very solid outing on Wednesday, striking out 2 in 1.2 innings and topping out at 98.9 mph:

Codi Heuer passed another test tonight for the @IowaCubs.

More than an inning of work, scoreless outing.

1.2 IP, 1 H, 0 R, 2 K. Last fastball struck out Luke Voit at 97.5 mph. Topped out at 98.9 mph.

He is a bullpen-changing arm.

— Alex Cohen (@voiceofcohen2) May 25, 2023

“We’re excited to get him back,” Hoyer said. “When that is exactly, I’m not sure. … Once we feel like he can definitley help us up here and it’s the right thing, we’ll bring him up. It’d be nice to have his experience coming out of the bullpen.”

The 26-year-old right-hander has 86 career MLB appearances under his belt with a 3.56 ERA, 1.20 WHIP, 23 holds and 3 saves. The Cubs acquired Heuer from the White Sox in the Craig Kimbrel trade in July 2021.

There’s also the potential of a rebound from the offseason additions.

Boxberger does not have any structural damage in his arm and the Cubs are confident he won’t be out long. 

Fulmer, meanwhile, has been the victim of some bad luck (the opposition has a .370 batting average on balls in play against him) and the team believes he can turn it around.

“He’s been pretty exceptionally unlucky in a lot of ways — his walk to strikeout ratio is pretty good,” Hoyer said. “I have very little doubt that he’s going to turn it around and have a really good year.”

As the Cubs look to change their fortune in close games — and on the season as a whole — they know getting the bullpen right is a huge priority.