As a teacher:
"You can lead a horse to water, but you can't make it drink."
That being said, when my students struggle, parents will blame either me or the student. Within the school, it's on the teacher's onus to make kids successful...
If the mistakes are mental mistakes, then that's on the player. If the coaches aren't telling players what to do correctly, then it's obviously on them.
From the outside, how do you assess who is at fault between player and coach? You would like to think that to coach taught the player to do X in a Y situation, but the player does Q. I'd say based on the Rivera coaching term, it's on the players. We've had more success in the past running a similar game plan. They aren't executing them properly, which should fall to the players.
On the flip side, there has been a lot of speculation that the team didn't work very hard this off season. That's the coaches and players there. If the rested on their laurels, then it's time to go head hunting.