I wouldn't put too much stock in that game, it was a dead rubber. I think it was just a bad signing. He didn't really show that much in the Dutch league to suggest he could come to United and be a completely different player. He's one of those where he has a good touch, is of a good size and can occasionally play a decent enough pass where you think "if they put it all together, they can be something". I'd say the bigger part of being a top midfielder is the mental aspect, knowing where to move, where to sit, how to open yourself up to receive the ball and get on the ball constantly and how to consistently dictate the game. Van De Beek never showed that ability. He's not a player who ever dominates the touches or is a playmaker, he's just a link up player. He's not a player who can progress the play well through carrying or dribbling, and he's not some sort of beast with his work rate or pressing either. I just think it was a signing where we thought he'd be a lot better than he actually is, and so Ole doesn't feel the need to play him much.
With Memphis, I don't think it was an expectation thing either. When I watch Memphis now I'm still not that impressed. He's turned into a smarter player and makes better decisions, and has a good shot on him, but he was never much of a explosive winger. I still don't think he's a player who's good enough for a top team. The tier below the top teams is where I'd put him. Spurs, Arsenal, etc. Barca getting him is just a further marker into how they are poorly run IMO.
Martial on the other hand, I think never kicked on mentally. Didn't have the drive or hunger to take the next step. He showed all the ability in the world at the start, but then somewhere along the way just became quieter as a player. His control isn't as good as it used to be, he looks much slower, and he looks weaker. 19/20 he had a good season but I still think he didn't show the same level of raw ability that he did at the start.