Greenwood, who went on three Lions tours as a player before embarking on a career in the media, reckons England captain Robshaw has proved he would be a more than able deputy if Warburton is injured or overlooked.
The Harlequins flanker led his club side to a first-ever English title this year after guiding his country to an unexpected second-placed finish in the Six Nations thanks to away wins over Scotland, Italy and France and a convincing home success over Ireland.
"For me the candidates are Warburton and Robshaw," said 1997, 2001 and 2005 Lion Greenwood.
"It will be key for Sam to play a whole season. If he can play eight or so internationals for Wales he will be Lions captain.
"But if he suffers more injuries and only plays three or four it opens the door and you can't underestimate what Robshaw has done with England.
"I can't see (the coaches) going back to previous captains Paul O'Connell or Brian O'Driscoll.
"Rugby has really become a 22-year-old's sport and (they) should give it to one of them."
The last Englishman to captain the Lions was legendary Leicester lock Martin Johnson back in in 2001, while Wales haven't produced a Lions tour skipper since Phil Bennett took the reigns in New Zealand 35 years ago.
The last two captains of the Lions have both come from Ireland, with O'Connell and O'Driscoll leading from the front under Sir Ian McGeechan and Sir Clive Woodward.
O'Connell led the Lions in 2009 and is still among the favourites for a second row spot in Australia next summer, while O'Driscoll saw his involvement as skipper in 2005 ended by a horror tackle from Tana Umaga and Kevin Meealamu within the first few minutes of the opening rubber against the All Blacks in 2005.
O'Driscoll's hopes of following fellow countrymen Willie John McBride and Mike Gibson in becoming just the third person to play on more than three Lions tours were described as all but dead in the water by some critics following a serious neck and shoulder injury at the back end of last year but Greenwood is convinced the veteran centre still has a valuable role to play Down Under.
"Brian's probably just behind the blue ball in terms of selection for the opening Test team," added Greenwood.
"He missed virtually all of the season after getting injured at the World Cup. But he still does things that defy his age and against New Zealand he was magnificent in defence.
"Even if he doesn't get in the Test team there will be no one better to have in and around the camp than Brian O'Driscoll."