I HAVE seen them. If you see a injury notification and a player go to his knee, then that is an off-ball injury.
One other injury related note:
I DO suggest turning Pre-Existing Injuries OFF in your franchise menu. This seems to help with injury variety (though RBs are still heavily favored to get hurt - like real life). One thing...IF you turn Pre-Existing Injuries Off in the middle of a season, it will wipe out ALL injuries league-wide, so tread carefully.
Some people are also beginning to consider manual editing of RB injury ratings to compensate. Honestly, that's probably the best solution overall....injury sliders jack with gameplay too much and even that aside, lowering that slider will also lower injuries too much for other positions.