At Barca our Wingers play as Forwards (Wing-Forwards). They are not there to cross the ball, they are expected to score and assist goals. We do not play our Wingers across the midfield, like, say, Man Utd do, in a 4-4-2.
He had an assist against Depor and Milan, and would have an assist against Rayo too if the finishing was better. Consistently providing assists, combined with hard work and fantastic movement off the ball and the odd goal, that's actually a very good player. You can't expect every player to be a star and Sanchez does a lot of the dirty work. If he starts banging in goals as well then all the better.
Ask yourself this: What if we weren't already ahead and comfortable?
In that case the Sanchez and the team as a whole would have maintained a higher level of performance.