The former England star, who was part of the team that lifted the trophy in 2003, believes Mike Tindall and Shontayne Hape deserve their places in the squad.
New Zealand-born Hape has been under fire after a poor performance in the warm-up game against Wales in Cardiff, while the effectiveness of the Tindall-Hape axis has been criticised for a lack of pace and invention.
But Greenwood, a former centre, says England have a perfectly balanced side, with Leicester's Manu Tuilagi adding the missing ingredient.
"England are in pretty good shape," Greenwood said. "They've answered a lot of questions but there's still a few more tough ones being posed, as Australia, South Africa and New Zealand are lurking around the corner.
"Hape did some very good things in the Six Nations.
"We all want our centres or our wings to score 58 tries every game, jump through burning hoops and do triple salcos but rugby's not about that.
"It's about putting components together that balance nicely and Shontayne is a very good defender.
"He had a couple of bad moments in Cardiff, coming back from a knee injury, and it's quite harsh to write someone off on the back of one performance but it gave an opportunity for Tuilagi to come in and he took it.
"Tindall brings his experience and had an outstanding game in Dublin.
"He's a much maligned player, outstandingly good at what he does but he needs a counterfoil and in Tuilagi I think they've found one for him."
However, Greenwood, who ended his professional career in 2006, thinks England need extra cover in the back line.
He said: "I'd have taken an extra winger - an out-and-out winger.
"If Chris Ashton or Mark Cueto get injured, I'd like to have a genuine recognised flyer to cover a genuine recognised flyer.
"I would have taken Charlie Sharples as he's an outstanding defender as well.
"A lot of the time, out wide, you need to be in total control of what's going on, see the game and work with someone like Tindall.
"Sharples is great at the breakdown and is the fastest thing on two legs so go for pace, pace and a bit more pace."
England kick off their World Cup campaign against Argentina on September 10, and Greenwood is confident Martin Johnson's men can beat the side that finished third in 2007.
"Argentina have lost a little bit of their stardust," he said.
"There will be no Ignacio Corleto, no Juan Martin Hernandez and no Agustin Pichot.
"They've still got Felipe Contepomi but he's had some injury problems so is not quite the player he was.
"The pack, with people like Mario Ledesma and Rodrigo Roncero, is still world class, but they peaked in 2007 so as an England fan you'd like to think they can't peak again.
"They'll be unbelievably difficult opponents but if England can maintain patience and maintain accuracy, there's too much pace within the team as a unit to allow Argentina to really trouble us and defeat us."
Greenwood is confident England can go all the way to the final but thinks hosts New Zealand will finally get the monkey off their back and lift the Webb Ellis Cup.
He added: "England are the highest ranked northern-hemisphere team, which is fair enough as we won the Six Nations.
"We've beaten Australia home and away in the last 12 months and we've got a great chance.
"We've got an enticing group but England have got to believe they can win that group. They can't have any doubts about that.
"We can turn Australia over in the semi-final. I think we can make the final but New Zealand are destined to win this one.
"It's tough enough winning a World Cup and to do it in New Zealand is doubly so, but I think we've got enough hard-nosed stubborn players who'll back themselves against anyone."
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