Green scored 13 goals as the Stags just missed out on the League Two play-offs.
But the 30-year-old only managed four league goals after Steve Evans took over as manager in November.
Green told BBC Radio Nottingham: "He's the new era and unfortunately I wasn't part of his plans; that was obvious. I turned into an unhappy player."
Green, who returned to Mansfield in May 2015, scored 86 goals during two spells over four seasons with Mansfield, but said the time was right to leave.
"It is demoralising for any forward if you have scored the amount of goals I have and you are not getting the respect you think you deserve," he added.
"Bids were being turned down and they were not really enlightening me about it, so it was very frustrating towards the end of the season."
Mansfield agreed to cancel his contract and Green said there had already been plenty of offers.
"It's from League Two to the Championship at this moment," he added. "I offer goals, I am still quick and strong and I look after myself. It will be interesting.
"I have some great, great years at Mansfield Town and it gave a lot to me as a club and I gave a lot back. But I have to move on - that is football. I think my time at Mansfield was done.
"Everything changed as a positive for the football club when Steve Evans came in, but not for me. Sometimes you have to shake hands and move on in a situation like that. It was very friendly and respectful and ended on good terms."
Striker Matt Green says he is attracting interest from Championship clubs having left Mansfield Town following a "demoralising" season.