TORONTO – B.J. Upton and his majors-worst .148 batting average were out of the Braves lineup Monday, but the center fielder was hardly getting a day off.

Upton spent most of the day working with hitting coach Greg Walker and assistant Scott Fletcher on a significant change to his swing, which has been a mess. Manager Fredi Gonzalez, who didn’t know if Upton would be back in the lineup Tuesday, said the emphasis was on getting his front foot down quicker so he’s not late on so many pitches.

Walker said that was only part of the adjustment they’ve asked Upton to make.

“He’s had three (problems with his swing) all year,” Walker said. “He’s back of center. Has bad posture, uphill posture. And he’s late getting his heel down. And we felt like all along that he’s been overloading, and he’s been stuck back of center.”

Upton went 0-for-3 with three strikeouts in Sunday’s 4-2 loss to the New York Mets, and on Monday at Toronto the swing project appeared to move to another level of urgency. At one point Upton was watching video in a room adjacent to the visiting clubhouse at Toronto’s Rogers Centre, with both current hitting coaches and former hitting coach and current first-base coach Terry Pendleton all watching with him and discussing aspects of his swing.

Walker said the key is excessive movement that Upton has used all year, “loading” from an upright position in his stance by rocking backward and then coming forward again. By that point he and his bat are at an upward angle and fastballs are often past him before he can complete the process to get his bat through the strike zone. And when it goes through the strike zone, it’s not on the even plane that gives him a better chance to make contact.

“So to get to center, to swing the bat and get his heel on the ground, he has to slide,” Walker said. “He has to slide, like, two or three clicks. And by then he’s late, and a lot of times he doesn’t ever get there, so he gets beat this way. It’s easy to see.”

With that in mind, Braves coaches have asked Upton to curtail much of the “loading” part of his swing, and to instead start from a set-back position where he is only coming forward into his swing, rather than first going back. They’ve spent hours in recent days going over video with Upton of his good swings last September, when he hit 12 homers in 108 at-bats with the Tampa Bay Rays and didn’t have all the back-and-forth swing load that he’s had this year.”

Walker said he wasn’t sure how or why Upton picked up the additional movement and load process in his swing during the winter, but the Braves didn’t want to ask him to make wholesale changes his first months with a new team, especially after he had a good spring despite the inefficiencies in his altered swing.

But now that he’s hit rock-bottom and stayed there through nearly one-third of the season, it’s time for change. Walker said Upton has been cooperative and worked hard trying to make the changes. What he was doing wasn’t effective, and recent minor tweaks and just “trying to use the whole field” didn’t produce results.

He’s hit .148 with just four homers, eight RBIs, 16 walks and 60 strikeouts in 155 at-bats, for a .236 on-base percentage and .252 slugging percentage. In his last 25 games, he’s 11-for-80 (.138) with one homer, three RBIs and 34 strikeouts.

Against right-handers, Upton has hit .136 with 44 strikeouts in 103 at-bats. With runners in scoring position, he’s a majors-worst 2-for-30 (.067).

Obviously, it’s not what anyone expected when he signed the largest free-agent contract in Braves history, a five-year, $75.25 million deal.

“He does several things that he didn’t do when he was a kid,” said Walker, noting the “bat twirl” Upton has developed, and the exaggerated foot turn. “But he did it last year in September, and he was the best player on the planet in September. So during the season, all we’re going to attempt to do right now is get him to a balanced, center position. He knows how to swing the bat. If you take that exact same swing, with bad posture from back, you’re going to miss back and under. And that’s where he’s been getting beat all year. And when he’s back there he tries to slide forward, but he can’t get there in time. He’s late. So get him on time and it will solve a lot of problems.

“When you’re trying to get someone to try to stop doing something, who’s doing a lot of things wrong, you try to come up with one simple that that will fix multiple things. We feel like if he’ll get in there where the fight’s at, in more of an attack posture, that his swing plane will clean right on up. To get to how he swung the bat in ’07 or ’08, which was a lot more efficient, it’s probably an offseason project. But right now, you can look on tape and say, OK, last year in September, when he hit 12 home runs or whatever, this is how you did it.”

Jordan Schafer was in the lineup in center field and batting ninth for Monday’s interleague series opener at Toronto.