Lots of concerns on the Morocco side of things before we get to the fact that they are facing Brazil of all teams:
1) Their coach resigned 3 months ago and his replacement is a youth coached who has never coached at the senior level.
2) Their best player (Hakimi) missed a month for a thigh injury and returned 2 weeks ago for the Champions League final. He played 120 minutes so I am not expecting any limitation, but the residual concern is that he joined the team late and had a special training program set up for him, so him and the team were not able to fully integrate/prepare in the weeks heading in.
3) Morocco's starting CB pairing (Diop/Riad) has never started a competitive senior match together.
So those are the Morocco problems. Now lets get into why this matchup in specific spells more trouble.
First off, Mazroui does not have the defending ability to defend Raphinha or any Brazilian skilled winger. There will absolutely be problems for them defending on the left side considering the likely starter on that side for Brazil is Raphinha, one of the most elite players in the world when healthy.
Secondly, one of the main weaknesses for Brazil has been set pieces. But, Morocco are missing some key pieces that have been their better aerial threats
The big risk of Hakimi/Diaz punishing the right side also gets lessened with the confirmed lineups having a makeshift CB at RB instead of Danilo as RB and attacking on the overlap, indicating Brazil is going to be more careful pushing numbers down the right side.
This is not the same Brazil that is stacked with legends like other world cups, but it is still Brazil. This is in their blood. And they have plenty of advantages in this one.