The reason the news about Jahvid Best's latest concussion didn't come until early Monday morning is because he didn't actually experience any symptoms until after the 49ers game, according to Jim Schwartz. During his press conference today, Schwartz shared that information and also made it clear that Best was available in the fourth quarter. He wasn't used because the Lions opted to go with Maurice Morris.
Considering Best is a bigger weapon in the passing game than Morris, it seems odd to me that the Lions would go with Morris in the final couple drives. That's why everybody figured Best was out with a concussion, but based on Schwartz's comments, it seems that Best was available and the concussion wasn't discovered until after the game.