Conte will use Sanchez as SS, and will give him a good run of games as starter. That role will suit him much better than winger, and its all about confidence and motivation for a player of his caliber. I think he will surprise a few people with his performances. Which is why I am a bit disappointed there is no option - that price is going to go high next summer.
Sanchez will start over Martinez, then it depends on form & performances. But that not a problem IMO, as Martinez is very young and still developing - a role as first backup striker plus rotation & sub appearances will give him enough opportunity to develop and shine.