The problem was not with Bastoni in my opinion.
We had two players alongside Bastoni who were playing in a position where discipline is not just required the most, but you need a really good amount of experience on top of that.
Just look at players who are superb at being CBs in a four man defenses, and get lost in three men defenses while still being primarily CBs.
Last season our wide midfielders were constantly helping our defenders, meaning most of the time De Vrij, Bastoni, Godin/Skriniar had back up all the time, on top of that we had real CBs playing at their position.
Yesterday even De Vrij or any given natural CB in the world would have had problems to play alongside Kolarov and Dambro, two players criminally playing out of their positions, while all this is being said, there wasn't the help from our wide midfielders yesterday like last season.
Not blaming Kolarov, nor Dambro, and especially no blame goes to Bastoni.
It was weird mix of circumstances that shouldn't even be option as a worst case scenario, hence get Skriniar off the transfer market. I'd rather have good CBs in the bench, than player divas who won't help in defense alongside players playing out of their positions.