He is not coward. First of all, a lot of people knew from the very start, that Mata won't be Mourinho's preffered player, because of his low defending workrate. He started very few games, although I'm sure he did his best on every training session. Even when he started he was playing outside of his preffered position, which is CAM. Even under AVB, when he was playing on wing, he wasn't doing well. He is 100% CAM. He was played on wing by Mou where he couldn't show his true potential. There were games where he actually tracked back, tackled and helped in defense, but was still taken off in the same match. Mata was never given a real chance. Also, I believe even by playing most games out of position he still managed to create more chances than Oscar. Mata is an introvert, deep person and he surely sees a lot of importance in his relationships with people. Mou is complete opposite - ruthless, goal-oriented extrovert, who doesn't care how he gets what he wants. Mou just destroyed his morale. Mata started believing that not matter how hard he trains, his contribution won't be appreciated by Mou. It's better for everyone that he left and I'm sure that if Mou talked to him and gave him a real chance, this move wouldn't happen.