In the first few games, he's shown what was expected of him. He can keep the ball, has an extensive range of passing, and has started to control the shape of the team with his organisational skills. He seems like an utterly professional and potentially crucial addition to the midfield. He returned to action relatively lacking fitness, but appears to have built that up over the last six games, and there's no hint that he is ready to sit out matches as he did at Bayern Munich. For all the talk of his age, he is barely 31, and if he is over his susceptibility to regular injuries, he's going to vital for at least two more years at Old Trafford.
Schneiderlin is the natural partner to Schweinsteiger when United play with two holding midfielders. His passing is nowhere near that of Schweinsteiger's, but it's more than good enough for the role. His tackling, fouling and regular short passing fits with the need to defend better across the pitch (despite failing to track a run for a goal against Swansea - hope he doesn't catch Carrick's brainlessness in that regard). Additionally, his mobility means that he can cover ground where Schweinsteiger can't.
Herrera is an odd case. He is an attacking midfielder in a side that doesn't use attacking midfielders. The options open to him are to play as a number 10, a controlling midfielder in a two, or a right winger. He can do all of these roles capably, but none of them make the most of his talents. His eye for goal, happiness in the tackle and propensity to appear in goalscoring positions make him ideal for a 4-4-2, or to play in a 4-3-3. Should United switch to such a system, he should improve his contributions.
It appears that Carrick has been phased out of the side earlier than most expected. Van Gaal hinted at a changing role for the midfielder with the arrival of Schneiderlin and Schweinsteiger, but he now seems to have been relegated to a substitute's role, but a very particular one. He appears guaranteed to feature in the last half of the match regardless of the situation - when chasing the game, he is expected to spend most of his time in the opposition half and build attacks. When United are defending, he plays his usual role of sitting deep and attempting to kill off attacks by screening the defence. Like most of the midfield, he's being held back by the poor performance of the attack, which in turn is not helped by two defensive midfielders being used when they are playing sides of limited quality.
Another player reduced to a sub's role. He is no longer being used as he was last season, a battering ram and an attacking menace from the outside. His partners on the left side of the pitch from last term, Ashley Young and Daley Blind, are not there anymore, and United have changed their formation. Expect Memphis, Herrera, Mata and Martial to be the attacking midfielders from the start of the game, and for Fellaini to function as a target man later in games. He can still be used to break up play when necessary, though, with his commitment to snide elbows and late tackles.
Who knows? He is not suited to playing in a two, and not strong enough to play in the middle of the pitch. It was expected he'd get his chance on the side of a midfield three, or as a winger further forward. He has the technique and pace to be worth a try, and his performances over the summer for Brazil showed a willingness to get forward. Under another manager he might achieve more this year, so it's up to him to force his way into the side. With such a thin squad, he may find his opportunities come in the cup competitions.