The Pittsburgh Steelers announced Tuesday they placed the tight end on the physically unable to perform list to start the season.
The move doesn't come as a surprise. Green hasn't participated in workouts after signing a four-year, $20 million contract this offseason.
NFL Media's Aditi Kinkhabwala reported earlier this month that Green was dealing with recurring headaches after suffering multiple concussions during his four years in San Diego.
The Steelers and Green insist his ankle has been the reason he's missed workouts, but the tight end's agent told Kinkhabwala the ankle is "fine."
Whatever the reason, Green will not be eligible to return until after Week 6.