In the immortal words of the great Allen Iverson, "Practice? We're talkin' bout practice, man. Not a game!"
In my playing days, I used to fake injuries all the time just to sit out practices. That never prevented me from being a shutdown corner. Never stopped me from shutting down Darren Charles in a game.
None of us know how Janis fared in practice, but in games, good things usually happen when Janis is in the game.
McCarthy should've played Janis over that sloth Allison. Barr's monstrous hit didn't end the Packers' season. Allison's fumble did. Odds are, Janis would've caught the rock in that instance and then, via his awesome speed, dashed into the endzone.