Luck's left knee clearly was down before he released the ball, meaning the play should've ended before his pass wound up in the midsection of Titans linebacker Will Witherspoon. Surprisingly, the ruling was not overturned, and the touchdown stood.
ESPN's Chris Mortensen said on "Monday Night Countdown" that the definitive replay proving Luck was down never was provided to the replay assistant or referee Pete Morelli in the 60 seconds of allowable access to the broadcast footage. As a result, there was no evidence to overturn the call.
Obviously, this would've been a much bigger story had the Colts not come back to win Sunday. It's a troubling communication breakdown nonetheless.