Coach Mike Tomlin did not specify the body part that was injured but classified the ailment as "just a day-to-day, minor injury. Nothing to speak of."
Brown was expected "to be fine in a few days," a source told ESPN's Adam Schefter.
Brown was not at Thursday's training camp practice at Saint Vincent College. He did attend Wednesday's session, watching from the sidelines and signing autographs afterward.
Brown has recorded an NFL-record five consecutive 100-catch seasons and is entering the second year of a contract that pays him around $17 million per season.
Other Steelers injuries include left tackle Alejandro Villanueva (arm/shoulder), linebacker T.J. Watt (hamstring), safety Sean Davis (groin) and safety Morgan Burnett (hamstring). Tomlin said he considered these issues minor.