There are particular question marks in midfield where apart from Manu Tuilagi, whose emergence was one of England's few positive notes from Down Under, there seems to be a spot for the taking by the time the Six Nations rolls around.
And former England centre Greenwood believes there are plenty of worthy contenders to be Tuilagi's partner when the players return to international action.
He said: "I think the centre pairing for the Six Nations is a tricky one and the Heineken Cup will tell us a lot.
"Farrell will be playing for England soon, there's no doubt about that, he's got tremendous mental strength and a great skill set, but there's others like him, like Joseph.
"We need all the test matches we can get and if England were playing on Saturday, I'd be tempted to have Flood at fly-half, Tuilagi in the centre and Joseph at 13, but Tuilagi's done his cheekbone [and Joseph is out with a shoulder injury].
"Riki Flutey's going well for Wasps too, so there are definite options, and Mike Tindall will need to start games regularly to fight for his position.
"Then if you look at other positions, you could go with Charlie Sharples. We need to make sure the fantastic talent that's out there in the Premiership is given an opportunity to flourish."
Farrell, 20, was instrumental in helping Saracens win their first ever Premiership against Leicester Tigers in the final in May, as his perfect kicking conversion rate at fly-half secured 17 points in a 22-18 victory.
Meanwhile Joseph, also 20, has been one of London Irish's standout performers this season, scoring four tries and starting all eight Premiership matches until he injured his shoulder against Bath.
Joseph may have to wait until January to play again but could still feature in Heineken Cup matches against Cardiff and Edinburgh.
Saracens' first European match is against Benetton Treviso at Vicarage Road on Sunday and the Italian club will be very wary of the threat posed by Farrell.
Greenwood says England should incorporate more young talent into the side if they are to retain their Six Nations title.
He said: "Perhaps because I was part of a team that won it, I felt then that change should be slower and we should bring in players to enhance us over a period time.
"Part of me now goes actually, you've got to have Youngs, Flood, Tuilagi, Joseph, Sharples, Ashton, Foden, something that makes you think 'wow, look at that'.
"There's tremendous talent out there, and it's now going to be a case of getting the right combination.
"There has to be a couple of players who've got at least ten caps in the side, you can't just throw in 15 newcomers, and if you get that right balance, and I'm convinced we can, then I think we can attack the Six Nations positively, and put this World Cup behind us.
"We've got Wales and Ireland at home, wouldn't it be great to turn those two teams over who performed better than us at the World Cup and kick on from there?"
Despite the disappointing manner of their World Cup exit to France in the quarter-finals, Greenwood says the old guard shouldn't be written off yet.
But the ex-Quins and Leicester centre is adamant club form should be the deciding factor when the Red Rose XV is drawn up in February.
He said: "I think potentially there's some big decisions to make for some of the senior players.
"Moody has already retired and others are close to being pushed for their places now.
"There's no magic wand to wave over the England team, the key is that the players in an England shirt are allowed to do the stuff they do well for their clubs.
"If they step up into an England jersey and don't give you what they're doing at their clubs, then you mustn't pick them.
"If they give it and have that fearlessness, which you suspect someone like Farrell could have, then he'll be on the fringes and will be pushing for a place, and if Flood or Wilkinson slip up in an England shirt, the next fly-half will be ready and waiting."
Greenwood was speaking at a rugby master class event for children aged 5-16 in Dorking, Surrey.
Super Skills Travel, which he co-founded with Austin Healey, organises week-long family holiday events in Sardinia with personal coaching from the England legends.
Go to www.superskillstravel.com for more information.