Honestly you have to be ready for any athlete to either suddenly lose it, or start a cascade of injuries and never to return to what he was.
Mike Brown played every dang game and snap for the Bears and NE for years. Then he signs a rich contract after his rookie deal is up, bam! Suddenly he's Mr. Glass and both of his legs systematically explode over several years with various injuries.
Tracy McGrady was unstoppable, then suddenly he tweaks his back and he slopes downward hard. Dwight Howard was every bit as good as LBJ, now suddenly he's still really good but not in the same conversation. Brandon Roy was a top 3 SG (maybe the best depending on who you asked) in the league. Yao Ming, Michael Redd, etc, etc, etc.