Brown will join the team for Sunday's 1:35 p.m. ET series finale at PNC Park.
Brown, who played in 144 games last year, will mostly play right field when seeing his first time in the big leagues this season. In 52 games with Triple-A Lehigh Valley, the 27-year-old left-handed hitter hit .257 with two home runs and 26 RBIs.
"He'll get some chances to start," Sandberg said. "I'll try to mix and match and rotate the guys as I see fit."
Ruf, hitting .235 with a .279 on-base percentage this season, said he was shocked by the move.
"You never know with things like this," Ruf said. "Hopefully I can go down and swing the bat well."
Ruf has served as an on-and-off starter recently. After starting the first seven games of May, the 28-year-old first baseman started in just six of the 19 games since.
Sandberg said the intent of sending down Ruf is to get him more steady at-bats.
"It is what it is," Ruf said. "Everyday at-bats don't really matter if I'm just going to come up and resume the same role."
John McGonigal is an associate reporter for MLB.com.