Mason (knee) was placed on injured reserve during the 2016 season.
Mason hurt his knee during the Bears' preseason finale and was waived/injured shortly thereafter. After clearing waivers, he reverted to the Bears' injured reserve list and stayed there for the entirety of the regular season. It isn't clear if his knee is still an issue, but Mason will need to prove his health in order to land a deal with another team in advance of the 2017 campaign considering his contract with the Bears expired at the end of the season.
Mason left Thursday's game after sustaining a knee injury, his return is questionable.
It's unclear how severe Mason's knee injury is, but given that he wasn't immediately ruled out of returning it likely isn't very serious.