MIAMI — One of the majors’ least effective starters is set to rejoin the Giants’ rotation.

Tyler Mahle, who was 1-7 with a 6.04 ERA when he landed on the injured list with a hamstring strain, will be activated Wednesday and start that night against the A’s.

Manager Tony Vitello delivered the news Friday before the Giants began a three-game set against the Marlins. All of San Francisco’s starters were pushed back a day following their rainout Thursday, meaning Mahle is slated to follow Adrian Houser on Tuesday.

Trevor McDonald, who took Mahle’s open rotation spot, will start Saturday against the Marlins, followed by Logan Webb in the series finale on Sunday. That means Robbie Ray will go more than a week since his last start in the first game of the Giants’ doubleheader Wednesday.

McDonald is 2-4 with a 4.64 ERA in eight starts.

Mahle’s return comes on the heels of one of McDonald’s worst outings, allowing four runs in 3 ⅔ innings in a 6-1 loss to the Cubs last weekend. Vitello did not specify how the Giants planned to incorporate Mahle into the rotation or the corresponding move to add him to the roster.

Mahle’s presence could be short-lived if he pitches well enough to generate interest ahead of the Aug. 3 trade deadline. His ERA was higher than any qualified starter when he landed on the IL on May 29, but the 31-year-old right-hander could represent a low-risk option for a contender to boost its pitching depth. He signed a one-year, $10 million contract this winter.

Acting on the advice of Bruce Bochy, who managed Mahle on his last team, the Rangers, the Giants signed him hoping he could replicate the 2.18 ERA he posted in 16 starts for Texas last over the course of a fully healthy campaign. He hadn’t thrown 100 innings since 2022.

Through 11 starts with the Giants, however, Mahle’s ERA is on pace to be the highest of his 10 big-league seasons. Opponents have hit .280 against him, also a career-high, and he has walked batters at his highest rate (9.6%) since 2020.

Those issues didn’t seem to be resolved when Mahle took the mound Tuesday in Oklahoma City to make a rehab start with Triple-A Sacramento. He issued five walks in three innings, though it only resulted in one run, while Mahle extended himself to 65 pitches.

Mahle chalked it up to a “weird” strike zone and said he was happy with his results.

He will build his workload up further in a bullpen session Saturday before returning to game action, Vitello said.

“That’ll be one,” he said, “where he’ll be able to get up and down multiple times.”