That's weak assumption, I guess. Casemiro is no longer at Real Madrid and he shouldn't be relied on to stop Barcelona alone. Under Ten Hag, Casemiro took different form because he was never the same as he was at Real Madrid. So what do you expect from Casemiro?
Ten Hag has the chance to win the match, and so does Xavi. I think it will be interesting, but definitely not El Clasico just because of Casemiro because you can never blame Casemiro himself when he failed to stop Barcelona.
Although this assumption is weak, I think that Casimero's qualities can make it difficult for Barcelona, although Erik Ten Hak does not rely solely on him, because football is collective and has a role for each other. But at least he understands Barcelona's playing style, even though he is no longer playing at Real Madrid. There is no doubt Casimero's experience in big games, because he has already played for Real Madrid and that's why I said Erik Ten Hak needs Casimero.
Both of them have the same chance to win the match, Barcelona is superior because they are playing at home, but the strategy of the coach and the metal of the players is also quite important in a big match like this.
Control the game and score goals, at least that's the instructions from every coach including Xavi. Playing well is the way to create better chance of winning, even if they are not 100% capable of winning every game. You're only hoping for luck if you only rely on your assumptions.
The game strategy that Xavi often applies is to try to dominate the game and they will try to keep the rhythm in order to control ball possession, while Erik Ten Hak can defend and carry out quick counterattacks, although this strategy is also not necessarily successful. As far as I have seen United's playing style since being coached by Erik Ten Hak, they also often play short passes and also rely on speed.
It is indeed difficult to predict who will come out victorious in the first leg, especially since so far the two teams are in good form. In my opinion there is no luck in football, there is only chance and strategy.