Two of Boston’s most reliable bullpen arms over the years, Koji Uehara and Junichi Tazawa, were allowed to walk away in the offseason.

Red Sox President Dave Dombrowski traded for eighth-inning set-up man Tyler Thornburg last December. He hasn’t been on a mound since March 1 because of a sore shoulder. So Thornburg is on the disabled list, along with Dombrowski’s previous acquisition, Carson Smith, who is rehabbing from Tommy John surgery last year.

For all the excitement over Boston’s deep rotation and its relentless lineup, the Red Sox bullpen attracted only worry and questions.

But look at the gang now. Through Friday night, they were third or fourth in the American League in ERA (2.18), WHIP (1.04), home runs allowed (three), holds (12) and OPS allowed (.561).

“That entire group has done a very solid, if not an outstanding job,” Red Sox Manager John Farrell said last week when the Red Sox were still in town. “Once they get a little success, their confidence is starting to grow.”

That confidence begins with the main man, Craig Kimbrel – one of three bullpen acquisitions by Dombrowski who is actually pitching.

Kimbrel was good but not great last year (3.40 ERA, and a WHIP of 1.09, his highest since 2010). He said he got into some “bad habits.”

So far this year, he is in sync – 2.00 ERA, 0.67 WHIP and 17 strikeouts and two walks in nine innings; the one hiccup being the blown save Thursday when he gave up a leadoff home run in the ninth inning, tying the game 1-1.

Kimbrel proceeded to retire the next six batters he faced, the last five by strikeout, using a 96-98 mph fastball, and an 86 mph curve.

“He’s probably in the best spot he’s been, from a delivery standpoint, in the year-plus he’s been here,” Farrell said. “He’s been in his delivery so consistently, able to generate that kind of power without abnormal effort … he’s an elite closer.”

Without Thornburg – who is currently on a long-toss program – and Smith (ETA June?), Farrell has had to figure out who to use, and when.

“Once we got into the latter part of the first week of the season, roles started to define themselves a little bit more,” Farrell said. “There’s a lot of big-league stuff out there. There’s power. There’s the ability to match up … as long as they execute.”

For power, you’ve got right-handers Matt Barnes (95 mph average), Heath Hembree (94 mph) and Joe Kelly (98 mph). And to match up, Farrell calls on left-handers Robby Scott and Robbie Ross (and seldom-used Fernando Abad).

Barnes was anointed the eighth-inning reliever and has been solid in high-leverage situations.

He has shown a much-improved curveball, which plays well when he’s commanding his fastball. Barnes’ ERA went from zero to 3.12 when he allowed three runs last Tuesday, entering with a four-run lead.

Even with that one lousy night, Barnes WHIP is 1.27 (improved from 1.40 last year).

Hembree brings an 89 mph slider to accent his fastball. He seems to be gaining more and more trust from Farrell, with a 2.25 ERA and 1.13 WHIP.

Farrell was not pleased with Kelly’s performance in spring training. But Kelly (1.80 ERA, 1.00 WHIP) seems to be settling in.

It was thought that Scott stayed in the majors because Thornburg was hurt, but Scott (0.00, 0.67 WHIP) has been lights out. Farrell does not let Scott stay out long (six games, three innings), but that could change.

Ross was sick at the start of the year. He came off the disabled list and ate up 22/3 innings (two runs) on April 14 when Rick Porcello was knocked out early. He was more effective his next time out, and could be relied on a lot, like last year.

For depth, Ben Taylor appears to be the first option out of Pawtucket. He’s already had two stints with Boston (1.59/1.24). Brandon Workman appears finally to be getting back from Tommy John surgery. In seven innings in Pawtucket, he’s allowed one hit and one walk, striking out 10.

When (or if) David Price comes off the disabled list, that will mean a back-end starter could go to the bullpen. More reinforcements are expected in the summer, when (or if) Thornburg and Smith are ready.

Questions about the bullpen are being answered, worries alleviated.

Kevin Thomas can be reached at 791-6411 or:

[email protected]

Twitter: @ClearTheBases