Er well the simple answer: search for them in the search bar!! If they show up once they have retired as a player then you can offer them various roles e.g. coach, scout (what ever they may want to be) ... if they dont show up when you search for them then they have decided not to stay in football and they leave the game all together.
Del Piero nearly always becomes a coach, hes a great attacking coach in my team. As for Nedved i have never seen him become staff.