Steelers coach Mike Tomlin said Brown was undergoing tests after experiencing discomfort in his knee, putting his status in question for Sunday's regular-season finale versus visiting Cincinnati.
Brown, who has amassed 104 receptions for 1,297 yards and an NFL-leading 15 touchdowns this season, has yet to practice this week. He was listed as out due to a coach's decision on Wednesday before sitting out Thursday due to knee soreness.
Last week, Brown finished with 14 receptions for 185 yards and two touchdowns in a 31-28 loss at New Orleans. That performance pushed him over 100 catches for the sixth consecutive season.
Pittsburgh needs to beat the Bengals and hope for Baltimore to lose to Cleveland to earn a spot in the postseason.
Brown was named to the Pro Bowl team for the sixth consecutive season and has been selected as a First-Team All-Pro in each of the previous four years.
In his ninth season since he was drafted in the sixth round of the 2010 NFL Draft, Brown has racked up seven 1,000-yard seasons. He has 837 catches for 11,207 yards and 74 receiving touchdowns in 130 games, including 103 starts.