Under was already sprinting past him and Xhaka was still at walking pace. If you can't see how that's a head start I don't know what to say. Ok he wasn't behind him, he was about level, which is basically just as bad with Xhaka's pace. He can squeeze up without getting that tight to him. It's common sense to stay a yard goalside and push up if they play it short. When an experienced, slow player doesn't recognise that alarm bells start ringing. That's normally a mistake athletic, reactive players make because they know they've got the recovery speed to fall back on.