If Tanner is in fact concussed again (using "again" loosely, as it's more likely a compounding of the first--which is notably worse than suffering a second concussion after full recovery from a first), he should receive a hero's welcome on Saturday but should be done for the season. There's zero reason to risk a third concussion (I'm no neuroscientist, but the rule of thumb I've heard is that a 3rd concussion is where things start to have very severe long term effects).
Even if there were very strong evidence that he is notably better than Athan (after today, I don't think there is), or if there were any chance of making it to a B1G championship game (I guess ILL losing today would make it interesting), I'd really like to think PJ is the type of coach to make the best decision for Tanner's health.
All of that being said, if Tanner is healthy and was taken out today in favor of Athan, you need to: make it clear that Tanner *can* play but the decision has been made to hand the reins to Athan; or, accept that Kramer is QB2. You can't say "Tanner is concussed and is going to sit" to save face then double back in the event of injury or poor play and come out after halftime on Saturday saying "we lied, Tanner is actually healthy enough to play" (would have much worse optics than even the Tua hit).