Miami Dolphins quarterback Trent Green was knocked unconscious with a grade-three concussion -- the most severe -- and he'll undergo further tests on the career-threatening injury to determine his future.

Associated Press

Trent Green

Green has been ruled out of Sunday's game at Cleveland a day after he was injured against Houston. Coach Cam Cameron declined to discuss the possibility the injury will prompt the 37-year-old Green to retire.

"I don't think we really know the full extent, because there are some ongoing tests," Cameron said Monday. "Right now I just think there are too many unknowns."

Green was hurt in Sunday's loss to the Texans. He was motionless on the field before being carted away and taken to a hospital, but he flew home with the team.

A severe concussion in the 2006 opener sidelined Green for half of last season when he played for the Kansas City Chiefs. Following that injury, there were concerns about his durability when the Dolphins acquired him in June.

Cleo Lemon, who replaced Green at Houston, will make his second NFL start against the Browns. The Dolphins and St. Louis Rams are the only teams with 0-5 records.