Tigers chairman Lee Hagipantelis said Sheens had taken the decision of his own volition, with the club approached last week by the coach's manager, Chris Orr, to discuss a possible restructuring.
Sheens was appointed head coach late last year on the proviso he would groom Marshall as his successor for 2025.
"It was following that outreach that these conversations were undertaken, all with Tim's engagement and blessing," Hagipantelis told AAP.
"Tim has anointed Benji as ready to step up into the head coaching role. It's very amicable and professional as it should be
"Suggestions, which I've read, about us terminating or knifing Tim are demeaning to all involved."
Sheens will step aside at the end of this season with Marshall taking the jump into the top job 12 months ahead of schedule.
The 73-year-old Sheens had re-joined the Tigers in 2021 as a football manager and was installed as head coach following the sacking last year of Michael Maguire.
When announcing Sheens' return in July 2022, chief executive Justin Pascoe said the club's long-term vision was focused on "putting together a rock-solid coaching plan for the next five years".
The Tigers, who picked up the wooden spoon last season, have won just three games all year and are likely to finish bottom again in 2023.
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