Thursday's 1-1 draw with Swansea City signalled the end of a gruelling Premier League schedule for Redknapp's side, which saw them play three games in a week.
The result left them two points clear of the drop zone but Redknapp's attentions will now turn to the visit of Nigel Clough's side, who boast an exceptional record in cup competitions.
United reached the semi-finals of the FA Cup last season and are safely into the final four of the League Cup this term.
Redknapp revealed that he will likely rest senior members of his squad such as Bobby Zamora, Clint Hill and Richard Dunne as he seeks to balance his priorities
"We haven't got a big squad. We're working with 19 maximum outfield players," Redknapp said.
"A lot of the lads will have to go again on Sunday, for sure. Some of them lads who played [against Swansea] can't play again, no chance.
"It's difficult to be as strong as you'd like to be. Ninety per cent of the bigger teams will put their better teams out in the later rounds.
"A few years ago, everybody went for the cup. Nowadays, everybody's either trying to get into the Champions League, or Europe or staying in the division and everybody messes around with their teams in the early rounds."
Clough's men have already seen off Premier League opposition this term, edging Southampton in December's League Cup quarter-final, while also getting the better of Fulham and Aston Villa in last season's FA Cup run.
The United boss cooled suggestions that there could be another shock on the cards this weekend in London, insisting the likely changes could work in QPR's favour.
"People are almost expecting an upset on Sunday, but it takes an awful lot to go in your favour to get the results against [a Premier League team]," he told the club's official website.
"Traditionally, Harry's probably done it as much as anyone, teams rest a lot of the players they consider to be the ones that start for the first team.
"But the strength and size of the squads mean that the team they put out is essentially a Premier Legaue team anyway.
"Even if they rest a few, I don't think it'll make it any easier. In some ways it'll make it harder because they'll be fresh."
Clough will also be forced to change his line-up with striker Chris O'Grady recently recalled by Brighton and Hove Albion, while centre-back Chris Basham remains suspended.
