Murtough,was brought into replace John Harbin in the close season but has opted to return to Everton.
Said Ray Ranson to the CT: "We are sorry to lose John but he was given the chance to go back to Everton and we didn't want to stand in his way.
"He could have gone sooner but wanted to see through the work he had started over the summer, not least at the new training ground. He didn't want to leave us in the lurch and we thank him for that and the good work he has done.
"We have enjoyed working with him and we wish him all the very best. It is just a case of unforeseen circumstances - he is a Manchester lad and needs to be back up in the North West for family reasons..
"The manager and I are big believers that modern-day football clubs should be run with the aid of sports science and we are always looking to strengthen that side of things as well as on the pitch. "I will be guided by the manager and if he finds a replacement I will support him."