Sadler will be Newcastle's third reserve-team coach in just over a year.
Lee Clark took over from Tommy Craig in a bizarre switch that still hasn't been explained.
Craig left under a cloud as Glenn Roeder revamped his coaching staff, and the popular Scot returned over the border to take up a post at Hibs.
Sadler's official post at Newcastle was assistant academy director and goalkeeping coach, but the job is seen as third in line to the top job.
Quite a popular man is Mr Sadler, Glenn Roeder tried to take him to Carrow Road this week.