He appears a bit sharper this season than what I remember from the tail end of last season. However, he is definitely still missing something in his top speed running off the ball. Remember seeing that video the other day of Sane running with the ball passed Van Dijk like he wasn't there? I know Sane is rapid, but still, for comparison yesterday Van Dijk only needed to jog to recover back and tackle against a "full pelt" Sanchez. He just doesn't have the pace to be a centre forward and get away from defenders over anything more than a few yards.
All of his best work yesterday was as a no. 10, turning with the ball quickly and then making through balls in to whoever is making runs in front of him (yesterday, Mata). The big problem with this though, as demonstrated at times last season, is that Sanchez's best work is the same type of thing in the same areas of the pitch as Pogba's best work, and Sanchez often ends up stepping on Pogba's toes.
Sanchez also a couple of times again yesterday showed that he still has his very irritating habit of doing an excellent turn to beat a defender and get in to space, but then even when he then has good options to pass to he inexplicably decides to dribble back again in to touble, getting dispossessed in the process.
He's an alright option to have, but I still don't really think he's what United need and has added much extra to the team. He should be a bit better again next season, but he creates as many issues as he solves. I also echo the sentiments of those here expecting him to be basically dead on his feet again way before the season has finished.