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9/1/2013 3:45 P.M. ET

Headley's sore back keeps him sidelined

LOS ANGELES -- Chase Headley has not progressed as hoped from a sore lower back, and the third baseman was held out of the Padres' starting lineup for the fourth straight game on Sunday.

Padres manager Bud Black said he would unlikely use Headley off the bench on Sunday. Headley has been sidelined since he left Tuesday's game against the D-backs in the fifth inning after his back stiffened.

"The back is still bothering him," Black said before Sunday's series finale against the Dodgers. "He was making progress and that has sort of reached a plateau. He didn't quite make the progress in the next 24 hours that he did the first 24."

Headley had been heating up at the plate, with a hit in 10 of his past 12 games. In 117 games with the Padres this season, he's hitting .240 with eight home runs, 28 doubles and 36 RBIs.

Last year, Headley was the National League Player of the Month in August and won the award again in September. This year, he was limited to 21 games in August and hit .261.

Alexi Amarista started at third base on Sunday.

Layne, Robinson first to join Padres' expanded roster

LOS ANGELES -- The Padres' first wave of September callups arrived on Sunday, as left-handed reliever Tommy Layne and catcher Chris Robinson were promoted from Triple-A Tucson.

Padres manager Bud Black said right-handed relievers Brad Boxberger and Brad Brach would likely join San Diego as early as Tuesday, after the Triple-A season ends.

"We're going to be able to expose some players for evaluation and get guys more acclimated to the Major Leagues so they're not in awe," Black said. "The experience is invaluable for players with lesser service time. And they can help us win games."

Layne, 28, is in his fourth stint with the Padres this season. He's appeared in nine games, going 0-2 with a 2.45 ERA and six strikeouts against two walks. The Padres were short in the bullpen after optioning Anthony Bass to Triple-A on Saturday to clear room for Kyle Blanks' return from the disabled list.

"It made sense to get another arm here when we could," Black said.

Robinson, 29, had a brief stint with the Padres two weeks ago but did not appear in a game. He will make his Major League debut soon.

"It's great for him to get exposed to September," Black said. "He'll get in a Major League game. He's been a guy that has been through the trials and tribulations of a Minor League season. It's great to have him be part of a Major League team."

Bass will likely return to the Padres as early as Tuesday. Bass, Boxberger and Brach were each optioned to Triple-A last week, and are eligible to return to the Majors after Tucson's season ends on Monday.

Austin Laymance is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.