Yamal has been the material they need to get the game going for Barcelona, Flick shouldn't make any mistake by reserving this player if he's fit to play. The player is still growing and he needs to work more on his finishing to become one of the best if not a world class player.
Yamal this season has indeed become one of Barcelona's key players who is very influential. At his very young age, his performance has been so good and boost other players to play much better within him in the squad. It's just that some time ago because the position was not possible due to the injury he suffered. And in this match, even though Yamal did not score a goal, he managed to provide a good assist, his contribution is still good for the club.
When I look at the Barcelona squad I only see young players who are hungry to play football for the club and I believe is only Lewandowski that is the oldest player in the squad (I think). These players have all it takes and they play as one, not in some teams that they do play an individual football but the Barcelona team don't have that very special player that wants to always be on the spotlight and it makes them to start playing some selfish football, no not with the Barcelona team.
It took the young player (Yamal) time to get into the game before we started seeing him playing the way he is playing now.
Yamal did everything a good player need to do even if he didn't score, he made sure that the player who's in the right position to score had to get the ball, but we don't see that in some players, so I'd say Barcelona and Spanish national team are very lucky to have this very player, with time he'll be greater than what we're seeing now.