MESA, ARIZ. — With the rest of Oakland’s pitching staff struggling, the A’s best starter through the first week and a half of the spring is No. 1 prospect A.J. Puk.

The 6-foot-7 left-hander turned in another strong outing Saturday in Oakland’s 10-4 loss to San Diego, and manager Bob Melvin said Puk’s fastball registered 97 mph; according to scouts who were at Puk’s appearance last Sunday, he was throwing 90-93 mph his first time out.

The only run Puk allowed Saturday was unearned, thanks to a passed ball by Bruce Maxwell. “He’s been really impressive,” Melvin said. “He’s the one guy who’s been putting up zeros for us.”

The A’s are unlikely to bring Puk up before June; he has not pitched above the Double-A level. With a lackluster competition so far for the final three spots in the rotation, though, Puk will be a major temptation if he continues to dominate Cactus League competition; opponents are 1 for 16 against him and he has allowed one hit and a walk while striking out four.

Kendall Graveman is assured a rotation spot — he’s the projected Opening Day starter — but Saturday he allowed four hits, two walks and four runs.

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Graveman usually throws the day before every start, but he said with an off-day Thursday, he’d experimented with not throwing at all, knowing that there are five extra days off built into the regular season this year.

“I wanted to see how that worked out,” he said. “But for me, personally, I’m going to need to throw on those off-days. That’s just an overall feel thing, it’s not an excuse.”

Chapman back Sunday: Matt Chapman, who missed the first week and half of the spring with right hand soreness, will make his Cactus League debut Sunday against the Padres in Peoria, Melvin said.

The third baseman took batting practice with the team, and afterward, he said, “Playing mañana, rumor has it.” Chapman had a cortisone shot for a bruised sesamoid bone on Feb. 22.

Briefly: Reliever Ryan Dull, who has not thrown in a week because of shoulder tightness, will play catch Sunday. He is unlikely to appear in a game until mid-to-late March. ... Jorge Mateo, the promising shortstop/center fielder obtained in the Sonny Gray deal with the Yankees, ran the bases and he said everything went well. He could be cleared to play in a game within the next day or two after being held out with a left knee ligament strain. ... Minor-league starter Logan Shore no longer has any trapezius discomfort, Melvin said, but there is no date set for him to resume throwing.

Susan Slusser is a San Francisco Chronicle staff writer.

Padres 10, A’s 4

Notable: Stephen Piscotty hit his first homer of the spring, a two-run shot in the second. ... Bruce Maxwell hit a solo homer in the third. ... Reliever Bobby Wahl, who is coming back from thoracic-outlet surgery, gave up two runs and his spring ERA stands at 20.25. .. Marcus Semien is batting .429 this spring, and DH Brandon Moss, who has an uphill battle trying to win a roster spot, is hitting .400; Moss singled and walked twice Saturday.

Quotable: “It’s now. ... We’re going to better, there’s no doubt in my mind. ... We’ve got to turn the corner, we’ve got to pitch, we’ve got to get outs. No more messing around with new things.”

— Graveman on how soon the A’s need to address their pitching woes; the team ERA is at 7.83.

Sunday: at Padres, 12:05 p.m. Radio: 95.7 FM

— Susan Slusser