No one knows, but personally I am pretty convinced Sancho will turn up relatively soon, but it might be after the world cup.
It will help, though, to have Martial back (hopefully on a more consistent basis), since I assume Sancho needs players like that to play off of, rather than having Ronaldo, Rashford and Bruno as his closest players. Of course he still has Antony and Eriksen in the fold, but at Borussia he didn't have to solve as much on his own as he does here, even with Haaland up front he had more creative/interplay-based forces and runs around him. I don't think it has to do with possession based v counter attacking based play, but rather who to play together with – obviously combined with his apparent confidence issues at play right now.
Rashford shows the difference between night and day concerning the latter issue right now. Even when not in peak ball-playing and physical form everything is better when there's a sense of belief in your body. The struggles can still be there, but you're a much better player when you deal with struggles than get beaten by them. Hopefully the past seasons have helped Rashford grow as a person, but even if he gets to an age considered to be experienced in the world of football, (almost) 25 is extremely young as a human being and most people haven't figured themselves out by the age the average professional football player gets into retirement. Especially if it's the alleged mental health issue, which takes a long time to deal with if it's a recurrent type, and that might even be for the lucky ones among us.
Sancho came at a bad time for the club, but was seen as the hero we've needed for about a decade. I can imagine that not fulfilling the prophecy of the RW to save us all can do something to a player's mind. But this lad is even younger than Rashford and I am pretty sure he will turn things around sometime this season. How to unlock that is between himself, ETH and his fellow players, but it certainly is possible. The Bundesliga is not at the same level as the BPL, but he is much better than we've seen (on average) during his games. Hope it works out.
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For now I think it's good that ETH rotates between them. Both to keep them both in shape, but also to keep the pressure off of them a little. They don't have to be our stars right now, the burden can be shared until either or both of them show up on the consistent base necessary to be one of the main carriers of the team.
I wonder if Bruno could be part of the rotation – and of course it would make sense to rest Antony plenty, too – but captains (and let's be honest, Bruno is the real captain right now) usually get played a lot more by managers emphasizing leaders in the team. Simeone would never drop Koke, and ETH is very similar to him in that regard as he never dropped Tadic no matter how many times he lost possession. As a player type I would definitely try Sancho in Bruno's position, but I don't think we'll see this happening anytime soon...
For now Rashford is easily showing more, but in preseason and moments like the Pool-goal we've seen that Sancho could be a great asset or center piece to the machine to build in ETH's mind. But if they rotate their starting positions for the foreseeable future, would be the best, in my opinion.