Marshall, who held the chief engineering officer title at Red Bull, will join McLaren in January as technical director, engineering and design.
Only two months ago, when McLaren announced a restructuring based on three technical directors, that job was given to Neil Houldey.
After it became apparent that Marshall could join the team, Houldey agreed to step aside and become his deputy.
"Neil has always been part of the process," said Stella when asked by Autosport about the change of plan. "He's been in the conversation, he has actually played as a team player in this process.
"And when we told Neil that we had this opportunity, which was quite unmissable, we elaborated together the solution.
"There's so much to do in terms of engineering and design to create the right standards to design the fastest car in F1 that actually, we thought this is a very powerful combination with Rob, the more of the technical authority, and Neil running more of the day-to-day activities within the department.
"So definitely Neil was in the process, a very constructive process."
Source: Autosport