OAKLAND -- A’s manager Bob Melvin was serious when he said a decision on the club's final starting pitcher would come down to the wire.

Just hours before Thursday’s Opening Day matchup against the Astros, the A’s announced that left-hander Cole Irvin was included on their 26-man roster. Right-hander Daulton Jefferies and left-hander A.J. Puk -- the other two pitchers competing with Irvin for a rotation spot this spring -- were both optioned to the club’s alternate training site in Stockton, Calif.

Irvin, 27, scored a major upset in this rotation battle. Whereas Jefferies (No. 4) and Puk (No. 2) are among the A’s top prospects on MLB Pipeline’s Top 30 list, Irvin was acquired from the Phillies in January in a trade that was viewed as merely a move to provide pitching depth in the Minor Leagues. But after turning in a solid outing in his first Cactus League start in place of Mike Fiers -- who was placed on the 10-day injured list, retroactive to Monday, with a lumbar strain -- Irvin parlayed the opportunity into a chance to begin the year in the Major Leagues.

Irvin turned heads in camp by posting a 1.00 ERA in five games (three starts) with 18 strikeouts and three walks in 18 innings. The game that clinched his spot on the roster, though, is clear. That came on March 26, when he dazzled against a Dodgers lineup filled with most of their regulars. Irvin held the defending World Series champions hitless through five innings and finished with six innings of one-hit ball while racking up 10 strikeouts.

“Cole pitched great,” Melvin said. “You look at his numbers in spring, and he pitched great. It was a tough decision, especially with the way Daulton pitched. We’ll go with this to start. [Irvin] pitched pretty well, and I think the game that stood out is the game against the Dodgers. It was a difficult decision, but that’s the way we’ll go in the interim.”

Irvin will fill Fiers’ spot in the rotation and pitch Saturday against the Astros. Fiers, who has been throwing bullpen sessions over the past week, will continue to work out at the club’s alternate site and could return to the A’s in late April.

“He pitched a couple of innings [Wednesday]. Next time out, maybe three,” Melvin said. “Once we get to 80 pitches or so, we’ll be comfortable with bringing him back.”

Rosenthal placed on IL
The A’s will be without new closer Trevor Rosenthal to open the season. The right-hander was placed on the 10-day injured list (retroactive to Tuesday) with right shoulder inflammation. With Rosenthal getting a late start to Cactus League play due to a groin strain, the A’s were hopeful he would be ready to pitch Opening Day after appearing in four games and posting a 2.25 ERA.

Rosenthal is not scheduled for an MRI at this point, though the A’s training staff gave him a dose pack, which will keep him from throwing for five or six days.

“It’s right shoulder fatigue. It just wasn’t bouncing back very well for him after his outings,” Melvin said. “After his last outing, he probably felt his worst. We’re just kind of treating the symptoms as of now. Not sure of a timetable.”

With Rosenthal sidelined, the A’s will likely turn to left-hander Jake Diekman for closing duties, though newcomer Sergio Romo also has experience in that role and could be a candidate for save opportunities.

“Jake is the obvious answer,” Melvin said. “But if the eighth inning looks pretty left-handed, maybe we do a little more mix and match. We have an assortment of righties or lefties with different stuff.”

Left-hander Reymin Guduan was added to the 26-man roster in place of Rosenthal. Guduan, a 29-year-old non-roster invitee with Oakland this spring, drew praise from Melvin for his sharp work in the Cactus League. The lefty appeared in nine games and allowed one run with five strikeouts in 9 1/3 innings. He spent three seasons with the Astros from 2017-19, posting an 8.03 ERA in 32 appearances.

To make room for Guduan on the 40-man roster, outfielder Skye Bolt was designated for assignment.

Montas improving
Frankie Montas, who was dealing with a torn cuticle on his right middle finger in his final spring start, is on track to start during next week’s series against the Dodgers after throwing to hitters during Wednesday’s workout at the Coliseum. The right-hander wore a protective pad on his finger and was throwing his fastball in the upper 90s.

“Frankie threw great [Wednesday], and our hitters were raving about him,” Melvin said. “Had a good split. Good slider. He was pretty excited after that. We feel like there’s enough time for him and on a good pace.”

Montas, who was Oakland’s 2020 Opening Day starter, is in search of a bounce-back season after going 3-5 with a 5.60 ERA in 11 starts last year.