Further tests revealed Brown has a partial tear in his calf muscle, according to ESPN's Adam Schefter, who added that while it's "unlikely" he plays in Week 16, Brown is expected to play in the playoffs.
Earlier, Ian Rapoport of NFL Network reported Brown had a calf bruise and that X-rays were negative. Brown had two catches for 24 yards before exiting.
Brown has been durable throughout his career. He played all 16 games for three straight seasons from 2013-15 and 15 games in 2016, but the Steelers may need to turn elsewhere if this injury is serious.
He finished the 2016 campaign with 106 catches for 1,284 yards and 12 touchdowns and had 1,509 receiving yards and nine touchdown catches this year entering play Sunday.
Brown's speed and shiftiness in the open field have allowed him to develop into arguably the NFL's best wide receiver and an MVP candidate. The five-time Pro Bowler opens up the rest of the field for his teammates because of his ability to beat press coverage over the top and creates holes for the running game because opponents have to shift additional defensive attention his way.
The Steelers can't realistically hope to replicate Brown's production elsewhere, but they can look to the combination of Martavis Bryant, JuJu Smith-Schuster and Eli Rogers as pass-catching options.
Still, Brown is an elite playmaker who makes the entire offense more dangerous. His team needs him back and on the field to live up to expectations as a title contender.