Young has been relegated to the bench since Memphis has claimed the left wing position, and he can hardly argue. Though it was to his credit that he kept out Angel Di Maria, in retrospect he didn't actually have to do that much to keep his place. Young's performances last season were adequate, and that's not in a pejorative sense - they were what the team relied on for many goals throughout the season. Memphis has struggled recently, and it might be that Van Gaal elects to reintroduce Young for his reliability.
Mata has again been United's best forward so far this season. We all know he lacks pace, and perhaps doesn't contribute much by way of assists, but with Wayne Rooney and Adnan Januzaj to play with, it's perhaps no surprise that he struggles to find much to pass to. He's continued his goalscoring form from last season though, and that makes him essential to the team. His link-up play with Matteo Darmian has been encouraging so far, though the Italian needs to improve his crossing to make it worth pursuing.
Memphis impressed against Club Brugge and in the early stages against Newcastle, but so did Rooney. His decision making is poor, so far, and his record in one-on-ones has disappointed. Nevertheless, for a player so young there are some encouraging signs. He is one of the fastest players at the club, and with Luke Shaw behind him means there's a quick outlet for a counter-attack, finally. He is confident and willing to take players on, which nobody else in the side does. And there is his track record for PSV, which is sound enough to expect that eventually this season, he will start to come good. It might not hurt for him to share the burden of expectation with Anthony Martial, either.
There was a chance he would have featured more last season, but injuries and loans disrupted his chances. He made 14 league appearances and scored two league goals for Derby Count in the second half of the season when on loan, and Steve McClaren wanted to take him with him to Newcastle United, who have a proven track record for identifying talented youngsters. He's fast, he's, he contributes with goals, and he's finally in the first-team squad. He's probably, with Paddy McNair, the least likely player to feature this season, but with the squad so light, injuries and suspensions could help him.