Johnson pulled up at the end of a route and fell to the turf untouched. He hopped off the field and went to the bench. He sat with his head in his hands as trainers checked him out.
He got up after a few minutes and walked gingerly to the locker room.
Coach Gary Kubiak said after the game that he was told the injury was not severe, but he couldn't be certain until Johnson underwent tests on Monday.
"I don't think you ever know about the hamstrings until the next day," Kubiak said.
It was Johnson's second game back since missing six games after surgery to repair an injury to his right hamstring.
He had four catches for 97 yards Sunday.
Information from The Associated Press contributed to this report.