Roster a little short amid rule oversight

The A’s thought they would be able to call up four more players Tuesday, which would have expanded their September roster to 37 in the heat of the pennant race, but realized in the morning it was not possible.

According to the collective bargaining agreement, a player who’s optioned to the minors can’t be called up for 10 days even if his minor-league season is over, so the A’s were unable to

promote Frankie Montas and three others.

After Monday’s game, manager Bob Melvin said Montas would be one of the relievers used in Tuesday’s game, but the right-hander’s return is on hold for 10 days. He was optioned to Triple-A Nashville on Friday.

The A’s have staffers in charge of MLB rule interpretations, but general manager

David Forst took responsibility and said in a Chronicle interview that it was his mistake, adding the rule had been changed in the new CBA, which went into effect last year and runs through 2021.

Previously, a player wasn’t required to wait 10 days once his minor-league season ended.

The A’s had planned to call up two other pitchers, Chris Bassitt and Ryan Dull, and outfielder Nick Martini. Dull and Martini were optioned Thursday, Bassitt on Aug. 27.

Roster limits expand from 25 to 40 in September. Forst said in retrospect, when the aforementioned August moves were made, the A’s would have had to call up fresh pitchers anyway. Monday’s trade for pitcher

Aaron Brooks wasn’t made to compensate for the oversight, Forst said, because the rules issue wasn’t detected until Tuesday.

The four players will be added to the roster after 10 days have elapsed for each.

Pinder nominated: Chad Pinder is the A’s nominee for the Roberto Clemente Award, which goes annually to one big-leaguer who represents the legendary outfielder who died in a 1972 plane crash while attempting to deliver supplies to earthquake victims in Nicaragua.

Pinder works with needy kids, including those with autism, at the School of Imagination in Dublin and contributes to various A’s community events that benefit the team’s community fund.

Briefly: Reliever Shawn Kelley is shelved with a finger laceration and might not be available until after Thursday’s off day . ... Brooks, acquired from the Brewers for cash considerations, was attractive because of his 3.35 ERA with Triple-A Colorado Springs, a team that plays at altitude. “It’s a good time to be here,” said Brooks, in the majors for the first time since 2015 when he pitched for both the A’s and Royals. Asked how he has changed since his last big-league stint, he said, “A lot of mental stuff. Everyone who can make it here has the talent, but in order to stay here, you have to make adjustments to mature mentally, and I think for the most part I’ve done that.”