I don’t think we’ve had a string of overseas central midfielders play for us, so it’s a bit of a red herring comment IMO. We’ve not had many succeed or fail, we've just not had very many, period.
Excluding foreign nationals brought from British clubs (Semedo, Bannan, Abdi, FDB etc), it’s only Momo Diaby, Lopez, Pelupessy and Paul Corry off the top of my head in the last decade. Maybe a few short term loans I’ve forgotten.