Redknapp witnessed his team, that included Robert Green, Esteban Granero and Park Ji-Sung, capitulate against the League One side, falling 4-0 behind before eventually suffering a 4-2 defeat in the FA Cup fourth round.
Redknapp told reporters: "Surely they are entitled to beat a team from two divisions down? It tells me, like I've said all along, that I need to improve the squad. That is why I have gone all over Europe these last three weeks trying to improve the squad because we are short.
"We are bottom of the league because that is where we are. We have picked up in the last 10 games or so, picking up 11 points, and we're getting blood out of stone at the moment.
"I know we need to improve and that is what we will continue to try and do this week, but it is not easy."
Redknapp took charge of Queens Park Rangers at the end of November.