Here is why Scott didn't take the job.
But Leitch has his sights set on succeeding McGhee at Aberdeen and believes his only chance of doing so is by impressing in the supporting role.
Leitch said: "They offered me the job at Motherwell and it was a really hard decision as I have spent many years there.
"But when Mark said I had the opportunity to go to Aberdeen with him I decided that was what I wanted to do.
"I probably would not get the opportunity to be Aberdeen manager unless I was here, working hard and showing them what I can do.
"If I can make the club appreciate what I do then I know I am giving myself the chance of one day being the Aberdeen manager.
"There is an element of selfishness from myself, but my long-term goal is to be the best manager I can possibly be and working with Mark can only help me.
"John Boyle and Motherwell have been fantastic to me as a player and coach and it was really difficult to turn down the manager's job at Fir Park.
"But I have a drive to do well here.
"Hopefully Mark and I will be a big success here and if Mark does go on to bigger and better things then hopefully I will have shown I am capable of taking on a job like this."
Leitch, who won the Challenge Cup in his one season with Ross County, is relishing the challenge of working with new players under a new regime and is eager to begin work with the players when pre-season begins on July 1.
He added: "Mark and I have worked extremely well in the last two years and I would like to continue the learning process with him for the next few years anyway.
"To come to a club like Aberdeen is a fantastic opportunity for myself. When I went back to Motherwell I knew all the players so it was an easy transaction for me.
"Coming here, working with a different board, with Willie Miller and for fantastic fans who demand success is an exciting challenge."