The Los Angeles Dodgers have become concerned enough about Yasiel Puig's nearly monthlong batting slump that manager Don Mattingly held him out of the starting lineup for Friday night's game in San Diego, the second time the outfielder has been out of the starting lineup in the past week.

Mattingly said he sensed Puig's frustration level has, at times, taken him out of his game.

The concern is that Puig's relentless style makes it difficult for him to stay consistent throughout a major league season. Last year, Puig came up in June and sparked the Dodgers' charge until batting .202 in September and having inconsistent at-bats in the playoffs.

"The season is long, and it wears you down," Mattingly said. "It's part of learning to regulate yourself here, as far as rest or anything else. We've seen Dee Gordon and how much more consistent his approach is day in and day out now, staying at a certain level. I think Yasiel's really emotional, and it's hard to be really emotional and play 162."

Puig did pinch hit for pitcher Dan Haren in the seventh inning of Friday night's 3-2 loss to the Padres and struck out against Nick Vincent to end the inning.

Entering Friday, Puig was batting .192 in his previous 20 games. He was invited to participate in the Home Run Derby at the All-Star Game, yet he has hit just two home runs since May 28. Mattingly said Puig is showing signs of frustration, often swinging at pitches well outside the strike zone.

Puig, who has played in 123 of the Dodgers' 135 games, was the player of the month in May, when he batted .398 with 25 RBIs. In June, he batted .248 with five RBIs. He led the majors in slugging percentage (.688) in July but has just two extra-base hits in August. Overall, he ranks 10th in the National League in batting average (.300) and sixth in OPS (.881).