The midfielder, 34, accepted the offer of assisting Charlie McParland in guiding the Reds' Under-21 side having completed his coaching badges earlier this summer.
Greening, who has been at the City Ground for two years, will juggle the role with his playing commitments for Billy Davies' side although he managed just six substitute appearances last term along with a month-long loan spell at Barnsley.
"I went away in the summer and did my UEFA A and B coaching courses, which the gaffer knew about," Greening told the club's official website.
"When I came back this year he asked if I wanted to become involved with coaching the Under-21s alongside Charlie McParland - I accepted straight away.
"I'm really looking forward to it. I definitely want to stay in football when my playing career ends and thanks to the gaffer I've got a great opportunity to learn from a very good coach."
However, Greening insists his playing days are far from over, adding: "I still feel like I've got a lot to offer. I feel really fit at the moment, I'm getting a good pre-season under my belt, I feel strong.
"I'm just going to carry on playing as normal but I'll also help Charlie at coaching sessions and matches so I can get some experience and learn from him and his staff."