NEW YORK -- In setting forth an agenda to improve the Mets this winter, general manager Brodie Van Wagenen identified four distinct areas he wanted to address. His work in three of those -- lineup balance, run production and bench depth -- has borne early fruit, with many of the Mets’ bats thriving.

The fourth area, the bullpen, has not been so rosy. Entering Monday, Mets relievers ranked 28th in the Majors with a 5.52 ERA and 27th with an .823 opponents’ OPS. The culprits have been everywhere -- even closer Edwin Díaz, who made a rare mistake Monday in serving up a go-ahead ninth-inning homer to Jesse Winker in the Mets’ 5-4 loss to the Reds at Citi Field.

Manager Mickey Callaway says he’s not worried, mostly because when the Mets give their relievers leads, they convert. Indeed, the Mets have recorded nine saves and 14 holds against three blown saves, a roughly league-average rate in a small sample size. Callaway’s gripe is centered more on relief struggles in games the Mets are already losing.

Perhaps Diaz put it best when he said Monday that “there are a lot of highs and lows with the bullpen so far.”

“But we are working really hard,” Diaz added through an interpreter. “Today, I actually thought we did a good job, but we have to keep getting better every day.”

Here’s a look at how Diaz and the Mets’ four other highest-leverage relievers have fared so far this season:

Edwin Diaz: 13 G, 11 2/3 IP, 8 SV, 2 BB, 20 K, 1.54 ERA
Winker’s home run aside, Diaz has been routinely excellent this season. He’s averaging 97 mph on his four-seamer and has fanned 45 percent of the batters he’s faced, while his 15.43 strikeouts per nine innings ranks sixth in the Majors (among pitchers with at least 10 innings). Before Monday’s game, Callaway referred to Diaz “as advertised,” which seems as apt a description as any.

“I’m ready for the highs and lows of the game,” Diaz said. “I pitched well, but I was ready to take this loss because I don’t think I’m going to have many more this season.”

Jeurys Familia: 13 G, 12 2/3 IP, 12 BB, 12 K, 5.68 ERA
Far more concerning on Monday was what happened during a scoreless eighth. Entering in a tie game, Jeurys Familia walked the leadoff man, hit the next batter and then, after a sacrifice bunt, intentionally walked another Reds hitter to load the bases. Next up was Jose Peraza, who pulled a 94-mph grounder to third, allowing Familia to descend the knife’s edge via a 5-3 double play.

When asked about Familia, Callaway mostly chalks up the setup man’s struggles to luck. But Familia’s batting average on balls in play is in line with his career norms, and luck doesn’t explain the walks -- 8.53 per nine innings, fourth most among Major League pitchers with at least 10 innings.

In particular, Familia has struggled to throw his signature sinker for strikes. He’s using that pitch less than ever, instead relying heavily on his split-fingered fastball -- a weapon he developed in 2014, then mostly abandoned before picking it back up again when the Mets traded him to the A’s last July. Which pitch mix is best? The Mets, who committed three years and $30 million to Familia, hope he finds the answers soon.

Seth Lugo: 13 G, 17.2 IP, 5 BB, 24 K, 4.08 ERA
Don’t be fooled by the relatively high ERA: Seth Lugo has been this bullpen’s savior. Most of the damage against him came early this season, when he was battling illness. Since April 7, Lugo has thrown 14 innings with a 1.93 ERA, 19 strikeouts and two walks. As usual, he and Houston’s Ryan Pressly lead their respective leagues in curveball spin rate.

Given all that, and given the fact that Lugo is a true fireman capable of pitching multiple innings at any time, the Mets would be in far more dire straits without him.

Justin Wilson: 9 G, 8.1 IP, 4 BB, 8 K, 3.24 ERA
Currently on the injured list due to left elbow soreness, Justin Wilson was inconsistent before suffering his injury. When he returns (perhaps as soon as this week), the Mets believe he’ll be just fine. In addition to throwing regularly in the mid-90s, Wilson ranks among the top six percent of Major League pitchers in terms of avoiding hard contact. The average exit velocity against him is 83.3 mph, also better than approximately 94 percent of the league.

Robert Gsellman: 13 G, 17 IP, 4 BB, 17 K, 4.24 ERA
What exactly is Robert Gsellman? Once a promising starter, Gsellman seemed to be developing into a dominant bullpen weapon as recently as last June. But he allowed three runs in a game on June 20, and he has posted a 5.40 ERA since that time.

Gsellman’s underlying statistics -- including his strikeout and walk rates, whiff rates and exit velocities -- indicate that more than anyone else on this list, he’s probably been the victim of some below-average luck. Still, the Mets won’t have infinite patience with Gsellman, whom they can option to the Minors if he remains inconsistent.