Mori and Robles aren't good enough.
It's not the massive mistakes it's just the sheer number of small needless errors that add up.
Robles passing the ball out 20 yards from goal instead of wellying it up the pitch lead to the first corner which he was rooted to the line for when it was 3 yards out which ultimately lead to the the equaliser as an example.
Deulofeu is in that category too.
Yes he set up the goal but everything else was shocking.
Mori is too weak for the PL at CB. Small players like Musa bully him out of the way and he doesn't have the pace to make up for this.