Both Mustafi and Xhaka were internationals with repute when they were brought. Mustafi was part of the German WC winning side, which was considered awei at the time. Most people on this site and others hoped that acquiring those two would solve our defensive and midfield issues at the time
That it didn't turn out as expected is not Wenger's fault. Happens every where. Who expected that Di Maria will flop at ManU or Diogo Forlan for that matter? Sometimes things click and sometimes don't.
Lichsteiner flopped here but did well elsewhere in SerieA.