Relief pitchers account for roughly one-third of all Major League roster spots. Because even the strongest contenders often have a weak link or two in their bullpen, the midseason trade market for relievers tends to be active across the industry.
That's particularly true in 2018.
Trade talks involving late-inning bullpen arms are expected to mature more quickly than usual this summer, helped along by two key factors: Trade Deadline buyers and sellers are clearer now than at a similar point in recent seasons; and the trade of Kelvin Herrera from the Royals to the Nationals established a reasonable price point for a two-time All-Star on an expiring contract.
Padres closer Brad Hand is a popular name among contending general managers, but you probably knew that already. With that in mind, here's an expanded list of relievers to follow in advance of the July 31 non-waiver Trade Deadline.
Blake Treinen, Athletics
Treinen never seized the closer's role over 3 1/2 seasons with the Nationals, but he's thriving in the ninth inning now. Treinen's SO/9 and SO/W rates are the best of his career by a substantial margin. He's comfortable pitching at the end of games (17 of 19 in save opportunities) and has prior postseason experience. Oakland could decide to move Treinen now, with his value at an all-time high and a significant raise coming in 2019 through salary arbitration. The Red Sox and Indians are good fits for Treinen, since their closers are due to hit free agency after this season.

Darren O'Day, Orioles
While teammates Zach Britton and Brad Brach have garnered more media attention as trade candidates, O'Day is outperforming both of them in 2018. O'Day has shown no sign of the control issues that have plagued Britton and Brach this season, walking only three batters and striking out 27 in 20 innings. A trade of O'Day would be more complicated than moving Britton or Brach in at least one regard: He is under contract for $9 million next season, whereas the latter two would be pure rental players.
Raisel Iglesias, Amir Garrett and Jared Hughes, Reds
The Reds' bullpen had drawn attention from Major League scouts long before the start of Cincinnati's surprising seven-game winning streak. The Reds' asking price for Iglesias remains high -- as it should be, for a possible All-Star closer with multiple years of control. Garrett has been a revelation in his first season as a reliever and struck out the side in Sunday's sweep-clinching win over the Cubs. Hughes, with a 1.36 ERA and under control through 2020, is probably the most likely of Cincinnati's relievers to be moved over the coming weeks.

Kyle Barraclough and Adam Conley, Marlins
The Marlins would rather move veteran Brad Ziegler, who has a 5.50 ERA after being bumped from the closer's role, but Barraclough or Conley would bring back more talent in return. Raisel Iglesias is 6 of 6 in save chances since becoming Miami's full-time closer earlier this month. Conley, a converted starter, has limited left-handers to a .136 batting average and zero home runs.

Joakim Soria and Xavier Cedeno, White Sox
The Braves are one possible landing spot for Soria, with Arodys Vizcaino now on the disabled list with a right shoulder ailment. Soria's contract includes a $10 million mutual option for next year, which may prove reasonable for a veteran closer enjoying the second-best SO/W ratio (6.40) of his Major League career. Teams need pitchers who are healthy and effective right now -- small-sample-size performers, acquired for a small-sample-size time of year. That describes Cedeno, a left-hander who did not debut with the White Sox this season until June 7 but has yet to allow an earned run in nine appearances. The White Sox could bring Cedeno back through salary arbitration in 2019, but the chance to move him at peak value now could be too alluring to resist.