Brown, 25, suffered the concussion on Tuesday, when he banged his head against the ground making a diving catch. He went for tests on Wednesday. By rule, a player must be cleared by MLB before being activated off the concussion DL.
In 99 games before the injury, Brown hit .271/.316/.531 (127 OPS+) with 24 home runs, the third most in the NL. He hit .236/.315/.388 (90 OPS+) with 12 homers in 147 games from 2010-12 before sticking this season. Brown was a first-time All-Star earlier this month.
Susdorf, 27, has hit .335/.419/.428 with one home run and eight stolen bases in 70 Triple-A games this season. The team's 19th round pick in the 2008 draft will be making his MLB debut.
To clear a 40-man spot for Susdorf, left-hander Jeremy Horst was transferred to the 60-day DL. Horst is out with an elbow injury that may be season-ending.