An injury to Green's left knee is what held him out of practice, though the severity and cause of the injury is unknown. There wasn't a point in Sunday's Week 2 win over the Chargers where Green appeared to be hurt.
According to Local 12, Green was wearing a compression sleeve on his left knee and during the portion of practice open to the media, Green did not report to the rehab field, which may be a sign that the injury is a minor one.
Whether he practices on Thursday and Friday will be more telling of how probable it is that Green plays on Sunday.
Green missed Weeks 6-8 in 2014 with turf toe and parts of Week 16 after suffering a concussion. He also sat out of the Wild Card round of the playoffs while battling that same concussion.
According to Geoff Hobson, early indications show the injury should not keep him out of the game on Sunday and due to the compression sleeve, it's likely just minor swelling. The Bengals will have two more official practices before heading to Baltimore on Saturday.