He would actually be a far less useless player for us as an out an out striker. That's definitely true for two obvious reasons.
The only competition for #9 is an oaf with no first touch and Rashford, who was abysmal for most of the second half of the season there.
And at striker he'll be on the ball a lot less than as a winger. Which means he'll have less opportunity to overplay and lose possession.
But if Sanchez at #9 is the ace in the hole for fixing our toothless attack, well, hello finishing 9th next season. Even if he'd be less useless, he's still toast as a Premier League level footballer in terms of his fitness. He might get a chance next season, have one good game, and then another muscle will pop.