Will Carroll's Black And Blue Report at Football Outsiders:
"Johnson's knee is thought to be the problem, though the team didn't crutch or brace him after the injury -- he was seen on the sidelines with ice -- and Johnson's initial reaction was to grab higher on the leg, near his groin. Still, Adam Schefter is dialed in and has consistently called this a sprained knee. Even a mild strain is going to keep Johnson on the sidelines at least a week."
If you're going to be away from the computer on Sunday, sit Calvin Johnson. The bye week isn't harsh on WRs, so you should have a decent option to start. Better safe than learning Sunday afternoon that Megatron sat.