We know the A’s will line up their starting rotation with some combination of Jesús Luzardo, Chris Bassitt, Sean Manaea and Frankie Montas for the first four games of the regular season. That quartet, however, still needs a fifth member for a complete rotation.

With veteran right-hander Mike Fiers expected to begin the season in the injured list due to the left hip inflammation that has kept him out of Cactus League action, the A’s have a vacant slot in their Opening Day rotation, and there are a few pitchers in big league camp still making their case this spring.

• Who might make the A's Opening Day roster?

Here’s a look at the pros and cons that will likely be considered for each candidate in the mix for the final rotation spot:

A.J. Puk, LHP
Spring stats: Two games (3 2/3 innings), 12.27 ERA, five strikeouts, three walks
Age: 25

Pros
Puk would be the ideal solution to fill Fiers’ spot in the rotation. He’s the A’s No. 2 prospect per MLB Pipeline and has already flashed his electric arm out of the bullpen for Oakland in 2019.

Cons
Working his way back from surgery to clean out bursitis in his left shoulder last September, Puk has run into some command issues early in Cactus League action, which seems to be even more cause for worry for the A’s than his decreased velocity. His fastball, which averaged 97.1 mph during his 2019 stint with Oakland, has been around 92-93 mph this spring.

What they're saying
“He was missing [location] by, at times, a wide margin, whether it was his breaking ball or fastball,” A’s manager Bob Melvin said after Puk’s outing against the D-backs on Monday. “It’s only his second time out, but he needs to command it a little better. As far as velocity, it is what it is. We’ll see if that ticks up a little bit. But the most concerning thing, for me, was him missing by a large margin at times.”

Daulton Jefferies, RHP
Spring stats: Five games (13 innings), 1.38 ERA, 20 strikeouts, three walks
Age: 25

Pros
The A’s original plan for Jefferies this spring was to use his opportunities in Cactus League games to rebuild some confidence to carry into the Minor League season and erase the sting from his rough debut last season against Texas in what was his only appearance of 2020. Confidence does not seem to be an issue in camp.

Jefferies has demonstrated the pinpoint command that led to him being ranked as the A’s No. 4 prospect entering 2021, and he’s done it facing some formidable lineups, too. Jefferies has gained a better feel for his breaking ball, which appears to be a mix between a slider and curveball, to give him another weapon when facing a tough situation. Gradually increasing his workload as the month goes on -- Jefferies tossed four scoreless innings with seven strikeouts and no walks in a 4-1 win over Colorado on Tuesday -- the right-hander should be built up enough to start out in the A’s rotation next month should the A’s decide to call on him.

Cons
Jefferies has only appeared in six games above Double-A in his career, including his one start in the Majors last year. Though he’s dominated his way through the Cactus League, the A’s could still want to get him more innings at Triple-A before graduating to the big league staff.

What they're saying
“He continues to work on his breaking ball,” Melvin said. “His changeup will always be there. He’s working on his command, which he’s always had, and just building up confidence as we go along. He’s in a position to continue to dig for a job.”

Cole Irvin, LHP
Spring stats: Four games (12 innings), 1.50 ERA, eight strikeouts and one walk
Age: 27

Pros
Irvin is the most surprising contender of the bunch. The left-hander was acquired in a trade with the Phillies back in January, mainly to provide pitching depth at Triple-A. But with each outing the A’s have given him this spring, Irvin continues to show more qualities that intrigue the big league staff.

Though he doesn’t light up the radar gun with a fastball around 92-93 mph, Irvin has made up for that this spring by showing good control of his four-pitch mix, which includes a four-seam fastball, changeup, slider and sinker. A’s pitching coach Scott Emerson noted early in camp that Irvin can “fill up the zone with strikes,” and that continues to be the case in games.

What has really stood out is the increased usage of Irvin’s sinker, a pitch he says he got away from after 2018 in the Phillies’ organization. He also has tinkered with his positioning on the mound, switching over from the first-base side of the rubber to the third-base side. That might not seem like a huge change, but Irvin said the switch has allowed his full body to free up, which helps him utilize the changeup and two-seamer more effectively.

Cons
Irvin has not fared well in 19 Major League appearances over the past two years, posting a 6.75 ERA in 45 1/3 innings. Perhaps this new form that showed up in Arizona for Spring Training can translate to the Oakland Coliseum, which is traditionally a good spot for pitchers to find the best versions of themselves. But with most of his appearances in Philadelphia coming out of the bullpen, the A’s could look to Irvin as more of an option for long relief.

What they're saying
“I’m just working with all the opportunities I get,” Irvin said. “I’m just getting back to what I know how to do and how I know how to pitch, and that’s using my sinker to complement my changeup. I’m excited.”