The 23-year-old back played eight times in the top-flight for Hull KR in 2016 and 2018, either side of being a member of their promotion-winning Championship squad.
He signed a long-term contract in 2018, but hung up his boots, aged 21, the following year, after a series of injuries.
He has now come out of retirement with Rams and Greenwood is excited about what he could bring to their side this year.
“He broke through as a Super League player a few years back,” Greenwood said.
“He made a bit of a comeback for the reserves last year and now we’ve got hold of him.
“He was good enough to break through as an 18- or 19-year-old into a Super League team, he is a bit older now and has sort of got over the hip injury that forced him to retire.
"If he can hit somewhere near his top form, which he was on a few years back, we have got a Super League player there.”
Greenwood tipped Samoan centre Bailey Liu as another potential star.
He said: “He came over from playing in the regional leagues in Australia and went to West Wales because he knew the coach there.
“The coach left straight away so he was left in a bit of a spot, stuck out on a bit of a limb down at West Wales.
“He has come to us, he wants to try his hand up here and he has got ambitions to play Super League.
“He has looked really good in training and I am looking forward to seeing him play.”
Rams can’t afford the high profile signings some other clubs have made, but Greenwood said: “We can get some guys in who’ve got the experience and you know they are going to be good, but we have always got to unearth these talents.
“People like our full-back Joe Martin, who was at Siddal before, as was our front-rower Tom Garratt.
"It is tough without your own academy and reserves, you have got to go looking in places other people don’t look, but sometimes you find these hidden gems.
"When they make it into becoming a regular Championship player, it’s good to see that.”