It's real simple, everyone sees that this guy has been breaking down towards the end of the season the last 2 years. It got so bad last year that they had to reinvent themselves on the fly and become a CJ Anderson-oriented offense.
He's got zero zip on the fastball now and relies on precise timing and extremely skilled receivers like DT, Orange Julius (though not anymore), and Emmanuel Sanders to make up for any of his physical shortcomings moreso than any other QB in the game.
Vegas lines haven't properly adjusted yet. I mean it took them all of 2014 to adjust to Nadal's decline (wasn't noticeable until May 2015 when they made him an underdog to Djokovic in the French Open, but he had already started doing things that he had never done in his career before, like getting swept by players like Berdych and Ferrer in straight sets)
The writing is definitely on the wall at this stage. By the time he returns for 2016 (if he does), he won't even be mentioned as a top 10 QB in any knowledgeable circle heading into that year (guys like Bridgewater and Russell will have pushed him out by then)