Steelers coach Mike Tomlin told reporters that Brown was sent to Pittsburgh to be evaluated for an injury that he declined to reveal, according to multiple media outlets.
Pro Football Talk, citing a league source, reported that Brown is dealing with a quadriceps strain. Tomlin said Brown is considered day-to-day.
The 30-year-old Brown had 101 receptions for an NFL-high 1,533 yards in 2017 despite missing two games with a calf injury. He also had nine touchdown grabs.
It was the fifth consecutive 100-catch season for Brown, who was named the AFC Offensive Player of the Year and received Pro Bowl honors for the sixth time.
Brown's 733 receptions for 9,910 yards are more than any other wideout since he entered the league as a sixth-round pick of the Steelers in the 2010 NFL Draft.