The centre or fly-half had been included in Brian Ashton's training squad for this weekend's clash with France but injured his knee playing for London Irish on Sunday.
Geraghty, who has impressed this season, was hurt in a collision with team-mate Fann Rautenbach and left the field before half-time in Irish's 22-13 win over Leicester in the Guinness Premiership.
Geraghty told LBC 97.3: "Things had been going really well for me; I'd managed to get myself fit after breaking my hand, I'd been called up to the England squad and we were playing some good rugby.
"To get a freak injury like that, to have the biggest guy on your pitch fall on your knee, is quite unlucky.
"I thought I'd be out for a couple of weeks - now it's six to eight weeks, which is a second blow, but I'm positive and am going to work so hard to get back as soon as I can, but not push too fast.
"I just want to be back playing as soon as possible."
Geraghty has backed fly-half Jonny Wilkinson to come good before the end of the tournament.
World Cup hero Wilkinson could face competition for his place after failing to live up his own high standards in recent outings against Wales and Italy in the Six Nations.
Geraghty added: "I look at Jonny and see he's got a great aura about him. In training when he says something everyone listens, everyone tries to take it in because what he says is pretty much spot-on all the time.
"To have him there, to learn from him - he was my idol growing up. I'm quite lucky to be training with him and I hope to play with him."