This sounds a lot like the debate over whether our championships prior to the Super Bowl actually "count". We may not have been Super Bowl Champions, but we were World Champions and that is what today's Super Bowl Champions are. Unitas may not have been the Most Valuable Player (because that designation did not yet exist), but, as "Player of the Year" he was certainly the most valuable player. IMO.
I do recognize, however, that if it was the AP's award to give and they did not give it to Unitas, then he was not "Player of the Year" and thus not the most valuable player in that year.