ARLINGTON, Texas — Sean Manaea’s velocity dip was something of a mystery most of the season, but the A’s left-hander said after an excellent final start that dramatic weight loss was probably a factor.

Manaea, who pitched into the seventh Thursday and allowed only an unearned run in Oakland’s 4-1 victory over the Rangers, said that he was prescribed attention-deficit-disorder medication during the spring and it affected his appetite to the point that he lost 25 pounds in just a few months.

“Weight loss was definitely big factor,” he said. “I know that’s definitely not healthy at all, and I noticed it halfway through the season when I was having a really hard time recovering, especially night games to day games.”

Manaea was diagnosed with ADD after team trainers suggested he get tested last offseason. The dosage of the medication wound up being an issue. “I couldn’t eat; food just wasn’t appealing to me at all,” Manaea said. “I’d take two bites, and I was full. It was bad.”

Next season, Manaea said he plans to pitch at about 240-245 pounds, up from the 230 he currently weighs. He weighed 255 last season.

A cortisone shot helped Manaea on Thursday: After missing his last start with upper-back tightness, he had an injection and it worked beautifully. Manaea allowed three hits, and he walked three while striking out six in 62/3 innings. “It’s finishing on a strong note for a guy who was really grinding toward the end,” manager Bob Melvin said.

Rookie third baseman Matt Chapman hit his 14th homer, a solo shot in the fifth off Miguel Gonzalez, as the A’s earned their seventh consecutive win over the Rangers. The A’s have won five road games in a row, their longest road winning streak of the year, and their 16 wins in September are their most in a month since going 17-9 in June 2014.

One of the most refreshing developments of September for Oakland is the team’s improved defense, but a nine-game errorless streak — tied for fifth-longest in Oakland history — ended in the fifth with Marcus Semien’s error on a grounder by Willie Calhoun. It wound up costing the A’s: Calhoun took second on a sacrifice bunt by A.J. Jimenez and he scored on Drew Robinson’s game-tying single to left.

The A’s regained the lead in the next inning with a two-out rally. Jed Lowrie singled and Khris Davis doubled before Ryon Healy sent home both men with a bloop base hit.

Manaea walked two and hit a batter in the seventh and left the game with two outs and the bases loaded. Liam Hendriks then got Elvis Andrus to fly to right. Hendriks hasn’t allowed a run in 18 of his past 19 appearances.

Oakland’s final run came in the ninth when Chapman delivered a single with two men on and no outs.

Manaea’s season was a wildly inconsistent one. He was 1-3 with a 5.52 ERA over his first six starts, 7-2 with a 2.92 ERA over his next 11 starts and 4-5 with a 5.55 in his final 12 starts. Overall, he went 12-10 with a 4.37 ERA after going 7-9 with a 3.86 ERA in his rookie year.

“There have been a lot more downs than ups, but that’s what makes you grow as a pitcher and as a person,” Manaea said. “Overall, the season taught me a lot.”

Bruce Maxwell, who began kneeling during the national anthem Saturday at the Coliseum, continued to do so Thursday at Texas — but, as the starting catcher, he was in the bullpen before the game warming up Manaea. He knelt in the bullpen, probably lessening the reaction at Globe Life Park; some Rangers fans in the crowd expressed confusion that they hadn’t seen any A’s players kneeling.

“Based on the fact he was in the bullpen, I didn’t really hear much,” Melvin said.

“I got boos, people yelling at me every at-bat,” Maxwell said. “Very mild, compared to social media. ... Not really a big deal.”

Maxwell returned from the concussion protocol Monday, and on Thursday, he took a foul ball off his face mask in the fourth inning but remained in the game after getting checked by a trainer. He said his chin took the impact and his jaw is sore, but his head is fine.

Oakland has won nine of its past 11 games.

Susan Slusser is a San Francisco Chronicle staff writer. Email: sslusser@sfchronicle.com Twitter: @susanslusser