LOS ANGELES - Jimmy Rollins was dropped to sixth in the Phillies' batting order yesterday, but he believes he has discovered the cause of his West Coast slump.

The shortstop was 4 for 27 in the first six games of the road trip against San Diego and Los Angeles, after his early-season issues appeared resolved. Rollins was batting .199 on May 15 but improved after that, boosting his average to .230 by Monday, the first game of the trip. He hit .292 (21 for 72) between May 15 and June 2.

But Rollins has struggled for the last week. His primary issue has been the most frustrating of outcomes for a hitter, the pop-up. Tired of hitting the ball weakly in the air, he retreated to the video room after Saturday's game, in which he started on the bench and finished 0 for 2.

Scrutinizing footage of recent games, Rollins discovered a mechanical issue occurring at the moment his bat made contact with the ball: His swing was flat, meaning that his bat dropped less than an inch at contact, causing him to get under the ball and lift it in the air.

"Hitting pop-ups, usually it's something right at the point of contact when your swing is coming off the plane," he said. "Everything is good right before the point of contact - your timing and everything else - but you're flattening out your swing right at that point.

"As opposed to keeping the angle of the bat above the ball, it might just get a little flat where it's level with the ball. When you finish the swing, if you kind of go down, that creates the underneath-the-ball effect, and you pop up."

Rollins then held his thumb and forefinger about an inch apart: "It's literally between here and right there, and that's a pop-up," he said.

He closed his fingers and continued: "That's a line drive."

Opening them again an inch apart: "And that's a pop-up. And it's just that little."

"When I saw it [on Saturday], I was like, 'Oh no, what am I doing?' " he said. "It's not like I'm late, not like I'm early. I'm just down. Now I've got something else to work on."

Manager Charlie Manuel, an expert on the science of hitting, trusted Rollins' diagnosis. "I think he's probably right, because he's underneath the ball," Manuel said. "Popping up is late. Underneath, and late."

Room for improvement

After three appearances following his reinstatement from a 50-game ban for violating the league's policy on performance-enhancing drugs, lefthander J.C. Romero said he was happy with the results but still honing his repertoire.

Romero pitched three total innings before last night, allowing three hits and one unearned run. "I'm OK," he said. "I don't feel like I'm in a groove yet. It's just consistency - putting hitters away when I'm ahead in the count, instead of nibbling. Having an understanding of what I want to do against certain hitters."

Romero also said that his change-up - a "feel pitch" that requires time to perfect - remained rusty. He admitted to feeling opening day-type nerves before his first appearance in San Diego on June 3, but not since.

"He's done the job, he's looked OK," said Manuel. "His command was a little bit off when he first came into the game [on Saturday], but he worked that out."

Stairs unavailable

Matt Stairs left the team before last night's game to attend his daughter's high-school graduation, leaving only Eric Bruntlett, Greg Dobbs and Chris Coste on the bench.

Contact staff writer Andy Martino at 215-854-4874 or amartino@phillynews.com.