I don't agree with your assesment of Xhaka's performance, but you have had the benefit of a second viewing. However, the fact that Xhaka sits in front of our CBs and can't win headers, ever, is why Sokratis was in two minds for the first goal (the fact that he let Eriksen run off him after that is all on the Greek though), and clearly nothing needs to be said about the penalty. The fact is with those two moments, Xhaka alone damn near killed our chances of winning this game.
Nothing more than that is really relevant. How you thought he played or I thought he played is unimportant to the question of whether or not he should be in the team. Mustafi had the same defence; "for 89 minutes he was great", he wasn't, ever, but some times he was good, when the game suited him, but one fuck up like yesterday and it ends the conversation for that match. Xhaka, like Mustafi, does this stupid shit every single game. It can't carry on.
And if there is no one else with the character and balls to step into his shoes and continue the policy of the holding midfielder facilitating our play, then the tactics must be changed. He's that much of a liability and a net negative.