You have to remember that coming into the Vuelta , Mas' self-confidence was pretty much at an all time low. He openly states that mentally he's not back to being his best yet, and that likely means he doesn't believe in his own abilties.
Here's the thing about Mas that is incredibly frustrating, and I say this as a fan of the guy. Physically he has all the talent to challenge for GT victories. His magic numbers are supposedly in the top tier elite. With him, a lot of his weakness has been mental. He rides like a diesel when he has the capabilty to launch massive attacks. His attacks the last two days were brutal. On stage 14, Lopez could barely hold his wheel (which probably led to Mas blowing up). Yesterday, that attack was impressive. Last year, he blew the race apart on stage 9, dropped Bernal and Yates, and only Roglic could match him. You also saw him crash on stage 14 and ride the rest of the race very tentatively. It's mindnumbing stuff to see him revert to steady pace after watching the rider who went so early on the l'Angliru two years ago ride so conservatively at times, but he's got to preserve the team's position but isn't confident enough right now to put in the attacks necessary to win the race.
Up until the Vuelta, his entire season had been a massive (no pun intended) failure, and his manager was quoted as saying he had not developed into a sole leader yet.
He's an incredibly frustrating rider to be sure, but I also think that he's overly criticized.