Because you are using a very small sample size to say that sanchez is doing well when messi is here while the truth is that sanchez started performing last season when messi went injured, for instance he had to wait for twenty something rounds before scoring his first league goal. When messi went injured boom! He starts scoring and making plays. In the start of the season he wasnt scoring that much. But messi goes injured boom! His goal talky rises! Messi is back he starts losing consistency. He doesnt score in the cup match against levante nor in the league game.The malaga game was an overral good performance by the team. Then messi is on the bench and he scores a double against levante, then a strange goal vs valencia then he becomes the old alexis missing sitters. There is a trend here!!