Peyton Manning was due to an injury. The point is that sitting out a season does not make a player lose his skills. Does it take time to shake off the rust, sure. But a 20 year old athlete will not lost all football skills for life because he sat out one season.
Jeremy Hill sat out last season for legal issues. He has shown this year that he didn't lose all of his skills by sitting out one season. Hill and Mathieu situations are very similar as it relates to football.
It doesn't have to do with skills, it has to do with the physical aspects of the game.
Everyone knew Peyton would be in peak physical condition because he's Peyton Manning. He has an almost 15 year track record of excellence and dedication. wasn't going to show up for camp 40 lbs overweight.
And with Hill, no one would have known about his condition of he was not ready. He just redshirts and we go with the other guys.
There are certainly going to be questions about what he has been doing with his free time. He already made the decision that weed was more important than football once, is it a stretch to believe that he might be choosing weed over staying in shape?