I really don't see him turning it around tbh, he was painful to watch most of the time last season.
People have been considering here whether him being 29 is indicative of him being too old or still theoretically peak condition, but this really varies a lot from player to player - age alone clearly doesn't tell the full story of where a player is in terms of their development, career and physical condition; body type and facial characteristics are also important to consider and provide a lot of evidence for where a player is. Look at Januzaj, for example, he is 23 but he still pretty much looks like a naive Bambi. However, with a few more years of toughening up he'll start to come in to his own and likely hit his peak pretty late. Young also still looks fresh-faced, despite being 32, and his continued pace and fitness is testament to that. Zlatan was also still clearly in pretty good physical condition when he arrived at United at 34, and he was very good until unfortunately taken out by injury.
Sanchez, on the other hand, looks like he's now turned in to a wrinkly old piece of leather. He looks like someone who has had a difficult life, started playing professional football at a young age, and would regularly bust a gut on the pitch. I think that it's now caught up with him and the clock can't be turned back.
It'd be scandalous if he kept getting picked ahead of Martial. I can only hope that the signs of Martial staying turn out to be correct, and are an indication that he's ready to demonstrate again why he should be on the pitch and Sanchez should be put out to pasture.