okay, so rugani should anticipate blatant mistakes too. that goal was on cancelo, no two ways about it.
i wouldn't sell him either because of his potential, but let's face it, from a purely defensive point of view, he's a mediocre footballer. that's why allegri opted to emre can covering cancelo against atleti, and that's why allegri also said that cancelo is an interesting player because he has goals in his legs. my guess is that with allegri remaining as our coach, cancelo would have played a lot higher on the pitch, even on the expense of costa or bernardeschi.
back to your original point: he's not a guarantee when the team is under pressure, especially not if he's our closest player to szczesny. he's a very good attacking player, he has every skill to become the next camoranesi for us, but as a defender, i have some questions about his consistency.