Uggla, who started in this year's All-Star game, said he Gonzalez told him of the change Sunday.
"He didn't really explain anything to me," Uggla said. "He just kind of said, 'We've got to make a change. I don't know how much playing time you're going to get the next 29 days.' "
Martin Prado started at second against Colorado on Monday for the second straight day.
"It's a situation where we feel like we need to do something for the team and the name on the front of the uniform," Gonzalez said. "It's hard. It's not easy. Two months ago, you don't even think about it."
Uggla hit only .233 in 2011, his first season in Atlanta. He finished strong with a career-best 36 homers with 82 RBI. He is the first second baseman in the major leagues with five 30-homer seasons.
Uggla started slow last season but recovered with a 33-game hitting streak and was the NL player of the month in August. He hasn't had a similar turnaround this season, hitting .160 in June, .115 in July and .207 in August.
"I understand what my numbers are. I'm not blind to that," Uggla said.
Uggla also said "Numbers don't mean anything to me. ... I'm part of the team that got us where we are now."
Gonzalez said he couldn't ignore Uggla's prolonged struggles at the plate.
"You just look at the big sample size," Gonzalez said.
Gonzalez said he hasn't considered the long-term implications of the move. Uggla signed a five-year, $62 million contract in 2011 that continues through the 2015 season.
"Next year is next year," Gonzalez said. "I might not even be around for next year. I don't even look that far ahead.
"There's going to be some mending of fences and maybe of relationships, but that's what we've got to do."
The versatile Prado has played left field most of the season. He also has made 18 starts at third base and could take over at that position in 2013. Chipper Jones is retiring after the season.
Gonzalez was Uggla's manager with the Florida Marlins before Atlanta. He said his history with Uggla made the decision especially difficult.
"Obviously, it's uncomfortable," Gonzalez said. "It's hard, especially with the relationship I have with him, six years with him. You want your players to be upset when they're not in there."
The manager said he expects Uggla to play a big role in the final month of the regular season.
"Knowing him, going down in September, he's going to have a big at-bat when he pinch-hits or he's going to have a big game when we put him in the game and it's going to get us in the postseason," Gonzalez said. "That's the way I feel about it."
Gonzalez says Uggla, who has 17 homers and 64 RBI, "absolutely" could win back the starting job.
"I wouldn't call it a permanent thing because it's not," Gonzalez said. "It's not like never again. You just never know."
For now, Uggla must adjust to his new bench role.
"The only thing you can ask is that people be up front with you," Uggla said. "He brought me in. He was honest with me. He said we've got to make a change."
Brian McCann also was held out of the lineup Monday for the second straight day. McCann was given a cortisone shot in his sore right shoulder after Saturday's game and could return on Tuesday.
Meanwhile, the Braves recalled right-hander Randall Delgado from Triple-A Gwinnett, where he was 4-3 with a 4.06 ERA in eight starts. He was 4-9 with a 4.42 ERA in 17 starts with Atlanta before he was sent to Gwinnett on July 15.
Delgado will pitch out of the bullpen.