The A’s brought one of their most reliable starters from the past three years back to the mix on Monday by reinstating Mike Fiers (lumbar strain) from the 10-day injured list.

The veteran right-hander was held out of Cactus League action due to a back issue that later developed into inflammation in his hip that required a cortisone injection. Progressing toward the end of Spring Training, Fiers began the regular season at the A’s alternate site in Stockton, Calif., and made four starts in scrimmage games against members of the Giants’ alternate site.

“We’ll figure out his role as we move forward,” A’s pitching coach Scott Emerson said. “Mike has been an important member of this team over the last few years. We liked the way he was throwing the ball down at the alt site. He’s ready to go.”

Fiers, 35, went 6-3 with a 4.58 ERA in 11 starts last year. Though he’s continued to see a decrease in fastball velocity over the past two years -- his fastball averaged a career-low 88 mph in 2020 -- he has consistently outperformed his secondary numbers. In 2019, Fiers went 15-4 with a 3.90 ERA despite a 4.97 FIP, faring much better pitching among the spacious outfield and foul grounds of the Coliseum than on the road.

It’s unclear whether Fiers will pitch as a starter or reliever for the long term this season. Whatever the role, he joins a pitching staff that has been on a dominant stretch. Entering Monday, A’s pitchers had allowed two runs or fewer in seven of the last nine games -- a 2.34 ERA in 77 innings over that span.

With Frankie Montas, Cole Irvin and Chris Bassitt listed as the A's starters for the final three games of their current four-game series at Tampa Bay, Emerson said Fiers will sit in the bullpen on Monday and is available to pitch in relief.

“If he doesn’t pitch in the next couple of days, you never know,” Emerson said. “He’s able to pitch tonight. We’ll play it by ear and see how the pace of these games go.”

To make room for Fiers on the active roster, the A’s optioned left-hander Adam Kolarek to their alternate site. Kolarek, who was acquired from the Dodgers in a trade in February, appeared in eight games and posted an 8.44 ERA.

“Adam had some days where he went a while without pitching. I think he needs constant work right now,” Emerson said. “I just want Adam to go down there and pound the strike zone. Get his ground balls. His last two appearances were pretty good for us. Sometimes, you just have to make tough decisions.”