Brian Cushing suffered a knee injury in the first quarter of Sunday's season opener against the Bears.
O'Brien said he would know more about Cushing's prognosis on Monday.
After walking off the field, Cushing was examined by team trainers and doctors on the sideline. He headed to the locker room and did not return. The veteran inside linebacker played one series and had two tackles before exiting the game. He was replaced by second-year linebacker Max Bullough.
"I don't know how long he's out so if he is out, we'll try and rally and make sure we do a good job until he gets back," J.J. Watt said after the game. "We hope he gets back as quickly as he can."
Check out the best #Texans arrival photos from the first regular season game vs the Bears.