Almost as if you don't follow football. De Gea has won everything bar the CL. Pogba has won all levels of titles bar the CL. Matic has won multiple titles. Sanchez has won multiple titles and cups, Valencia won multiple titles and cups, even Lingard has won multiple cups, Martial has won all bar the EPL and CL. I don't know who is left. If they aren't performing consistently then the coach needs to do something about that. Who are these unproven players that you are referring to?
Ok, let's take a look. If we define proven as consistent on the highest level, has won trophies, and has been a major contributor in those wins:
De Gea - fair enough, but he's the one player nobody is questioning is world class
Pogba - He won because he was on a team that dominated Italian football (and still does without him), and he was far from a key player there. He's on the French national team that hasn't won anything, and it's also arguable that he's a key player there
Sanchez joined literally 2 games ago
Valencia - Ok, but he's not a difference maker
Lingard - Won cups, and has made his contributions in those cup wins, but not a key player, and not even a consistent player yet
Martial - Same as Lingard
Lukaku - Proven goal scorer on a smaller team. Unproven in a big team, CL, or on international stage
Sanchez, Matic, and De Gea are the three players that qualify as "proven".
Let me illustrate my point further; take a look at the couple of games that Sanchez played and compare him to the rest of the attackers (Martial, Lingard, Lukaku, Rashford). He does all of the following
consistently throughout the game: control the ball, hold up the ball, get open, make simple passes, dribble without losing the ball, make penetrating passes, defensive work (pressure defenders, track back, tackle). None of the others (and you can throw Pogba in there too) do any of these things consistently, and for that reason I label them as unproven. If all of them at least put in the effort that Sanchez does, the team would perform exponentially better.
And to be honest, maybe Jose is wrong in trying to make Pogba into something that he's not, but nobody can tell me that Pogba is even trying. I mean look at his movement on defense; it looks more like pretending to play defense than actual defense. He's trying very hard not to catch anybody or get the ball. Not to mention the plain out ridiculous lack of decision making, when he constantly loses the ball in dangerous areas because he's trying to be fancy instead of playing it safe and doing the simple and logical thing.