League One side MK Dons provided one of the shocks of the fourth round with an impressive 4-2 win over Premier League strugglers QPR.
Redknapp lamented his side's defending for MK Dons' opening two goals and admits he desperately needs to bolster his squad before the transfer window closes if they are to improve.
"I thought we played well in the first half but gave two diabolical goals away and found ourselves 2-0 down," fumed Redknapp.
"We came out second half, their third goal was offside but that's no excuse, and the fourth was sloppy from us as well. Their first two goals gave us a mountain to climb and we couldn't climb it."
Asked if the team he put out was good enough to beat MK Dons, Redknapp said: "Absolutely, of course.
"Look at the team. We have got an England goalkeeper, a right-back from Man Utd, left-back Traore, Anton Ferdinand and Ben Haim.
"Granero from Real Madrid, Ali Faurlin, who you tell me lots of clubs want to buy, Ji-sung Park from Man Utd, Jamie Mackie and I have got Bothroyd and DJ Campbell.
"Surely they are entitled to beat a team from two divisions down, it's not a bad team."
Redknapp admitted the fringe players he gave the opportunity to impress failed to take their chance and that he may have to offload them and bring in replacements before next Thursday's transfer deadline.
"I gave them a chance because some of them have been knocking at my door saying they should play and with others I've been told they're good players. Well I gave them a chance and they blew it.
"You know what? It answers questions. Not for me - I knew the answers. But it maybe answers questions for those who didn't know.
"People think they should be playing and their agents think they should be getting a chance. Well, they had their chance.
"I've said all along we need to improve. That's why I've traipsed all over Europe trying to sign players.
"I'll go home with the raving hump. I didn't pick a team to lose, I picked a team that should have been able to beat MK Dons."