B.J. Upton and his majors-worst .148 batting average were out of the Braves’ lineup Monday, but the center fielder hardly got 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 said he didn’t know if Upton would be in the lineup Tuesday, said the emphasis was on getting Upton’s 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 (swing problems) 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 Monday at Toronto the swing project seemed to have moved to another level of urgency.

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 often are past or he swings beneath them.

“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 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 four homers, eight RBIs and 60 strikeouts in 155 at-bats. In his past 25 games, he’s 11-for-80 (.138) with one homer, three RBIs and 34 strikeouts. With runners in scoring position, he’s a majors-worst 2-for-30 (.067).

Heyward gets a rest: After playing all nine games since returning from a three-week stint on the disabled list following an April 22 appendectomy, Jason Heyward was out of the lineup Monday. Gonzalez said it was a good day to give him a one-day rest, since Reed Johnson hits lefties well and was 13-for-71 (.351) against Blue Jays starter Mark Buehrle.

“He’s played every game, every inning,” since the DL stint, Gonzalez said. “And we got (to Toronto) at 4 in the morning. Why not give him a day off? He’ll play tomorrow.”

Heyward was 8-for-34 (.235) with two doubles, three RBIs, six walks, seven strikeouts and .381 OBP since returning. He was hitting .163 with two homers in 92 at-bats for the season, including .143 (5-for-35) against lefties.

Etc.: Reliever Jordan Walden (shoulder) pitched one inning in a rehab game for Triple-A Gwinnett on Monday night at Syracuse, N.Y. Gonzalez said depending upon how Walden felt, he could have another rehab game Wednesday or be activated from the DL. The Braves look forward to getting him back. Cory Gearrin gave up three runs in the eighth inning to blow a lead in Sunday's 4-2 loss at New York that snapped an eight-game winning streak. … Brandon Beachy will make his next rehab start Wednesday at Class-A Rome. He's in the final stages of his recovery from Tommy John elbow surgery and could be back in mid-to-late June.