OAKLAND -- One day after the offense broke out for 20 runs, the A's bats could not rally back from an early deficit as they fell to the Marlins, 12-3, in Sunday's series finale at the Coliseum, putting an end to their season-high six-game winning streak.

Starter Joe Boyle lasted only one inning, a four-run frame for Miami in which he walked three batters and gave up a three-run blast to Nick Gordon before departing with a lower back injury. Boyle is scheduled to undergo an MRI on Monday to determine the severity of the injury.

"I felt it when I started pitching, and it got progressively worse as I went on, trying to grind through it," said Boyle, who threw 35 pitches (16 strikes). "Obviously, just didn't get the job done."

For the first time this season, the A's may need to test their pitching depth. Through Sunday's game, they used only 13 pitchers in 2024, the five-man rotation and eight-man bullpen the club set on Opening Day.

Notwithstanding Boyle's injury, Oakland's schedule will also include another new arm. A's manager Mark Kotsay said Oakland will appoint a pitcher as the 27th man for Wednesday's doubleheader against the Rangers.

Even before he exited Sunday's start, Boyle had been one of the biggest question marks in the A's rotation.

At times, the 24-year-old right-hander flashed the high-octane stuff that helped him win the fifth starter job out of Spring Training. Boyle entered Sunday with a 96.7 mph average fastball velocity, which ranked in the 92nd percentile among all qualified Major League pitchers. He's also struck out 29 batters in 27 2/3 innings, leading A's starters with 9.43 strikeouts per nine.

Boyle's main issue has been command, a trend throughout his professional career. His 23 walks in 27 2/3 innings lead the American League and are second only to the Dodgers' James Paxton overall (24 in 32 1/3 innings).

In his brief stint in the Majors in 2023, Boyle walked only five batters in 16 innings (three starts). But he has issued three or more free passes in six of his seven starts in '24.

"This start to the year hasn't been what Joe's probably wanted," Kotsay said. "We have seen an increase in the walks and lack of command with the fastball, which can be an issue. It kind of stands out right now, because for the most part as a staff, we haven't been giving out free bases."

Should Boyle be sidelined the next time his turn in the rotation comes up, the A's have multiple options down on the farm who could be called upon to start. Right-handers Osvaldo Bido and Joey Estes have Major League experience and are already on the 40-man roster.

Bido, who signed with the A's this past offseason, has gone 4-1 with a 2.59 ERA in six games (five starts) with Triple-A Las Vegas, striking out 39 against 13 walks. He made 16 big league appearances (nine starts) with Pittsburgh in 2023, recording a 5.86 ERA in 50 2/3 innings.

Ranked Oakland's No. 11 prospect by MLB Pipeline, Estes got his first taste of the Majors in 2023, allowing nine runs (eight earned) across 10 innings in two starts. Through his first six Triple-A starts in '24, he owns a 6.04 ERA with 26 strikeouts and 10 walks.

The A's could also opt to have one of their bullpen arms start a game if needed. Kyle Muller and Mitch Spence have both been solid in long-relief roles, though Muller struggled as a starter in his first season with the organization (8.10 ERA in 13 starts in 2023).

Spence, Oakland's Rule 5 Draft selection last offseason and the club's No. 24 prospect, pitched on Sunday in relief of Boyle. He allowed four runs on six hits and one walk while setting season highs in innings (4 2/3) and pitches (72).

If a need arises, Kotsay said, Spence would certainly be in consideration to make a start for Oakland. Primarily a starter in his Minor League career, Spence relishes the chance to get his first starting nod in the big leagues, though he added that his focus is on being ready in any situation the team needs him.

"It'd be great," Spence said. "I mean, the dream's always been to start in the big leagues, so it'd be an awesome opportunity."