I hate to reveal my sources, but google Marcell Dareus and see how many times you find mention of a lack of discipline, commitment, urgency, etc. "Buying in" can mean different things to different people, but I think McDermott spelled it out pretty clearly here:
"At times. I'm going to be very honest about it -- at times. At other times, not really," McDermott told Pelissero about seeing urgency out of Dareus. "This bus is moving and at the end of this year, we're going to have a pretty good feel for who's on the bus and who's not, and everyone knows that, I would hope. So, I would hope they've seen that from the moves we've made and the direction we're heading, the mindset we're embracing and the devoted mindset you need to have to win in this league. We're going in a certain direction for a certain reason, and we play and do things with a purpose. People that have that common purpose and want to share that common purpose, they'll be here."