CLEVELAND — Blake Treinen, who lasted just three weeks in the closer’s role for the Nationals last year, will head back to Washington for next week’s All-Star Game, representing the A’s.

Treinen, 30, was acquired in the Sean Doolittle-Ryan Madson deal with Washington last July. He has the best ERA (0.81) among major-league relievers, is 5-1 with 22 saves and has converted each of his past 19 opportunities.

“I’m pretty excited,” said Treinen. “To say I hadn’t been thinking about it would be a lie, but it wouldn’t be the end-all, make-all. A lot of us here have had really good seasons.

“I’m excited to go back, see some friends. It’s kind of cool. It’s where I debuted. It will be fun.”

Treinen was the only A’s player selected Sunday, but with injured Yankees second baseman Gleyber Torres also making the AL squad as a reserve, there is a strong chance that the A’s Jed Lowrie will be named as a replacement. Lowrie, who tied his career high with his 16th homer Sunday, is among the league leaders in many offensive categories and entered Sunday with the best fielding percentage among second basemen. (He made an error in the second inning, his second of the season.)

“We’re still waiting for Jed to get completely through, because he’s had one heck of a year,” Treinen said. “I’d let him take my place any day because his year is incredible.”

Khris Davis, who is second in the majors in homers since the start of 2016, with 106, also was left off the team. He told The Chronicle recently that he was open to participating in the Home Run Derby, but he had really wanted to do so only if he was on the All-Star team.

Treinen has been spectacular, working multiple innings 14 times, allowing only three of 14 inherited runners to score and allowing opponents a .172 batting average.

“I don’t know if there are too many relievers that have meant as much to their team as Blake has to us,” manager Bob Melvin said after Treinen’s two scoreless innings in Oakland’s 11-inning win Saturday.

Treinen also has turned into a team leader — and one who likes to have a little fun.

He has been changing the “P” in Stephen Piscotty’s name into a “B” on the whiteboard on which the lineup is written, though Treinen initially professed innocence when asked about it by a reporter who’d heard he might be the perpetrator from several sources, including Piscotty.

Told that Treinen’s denial was convincing, Piscotty said, “Maybe too convincing?”

On Sunday morning, Treinen fessed up to turning Piscotty to Biscotty for the past three weeks. “I was just messing with you!” he said, as he inked in the B.

Piscotty does not mind. “It’s a tasty treat,” he said.

Susan Slusser is a San Francisco Chronicle staff writer.