The London club rarely threatened in Sunday's clash as United won for the first time in a competitive match under Louis van Gaal, ensuring a miserable return to his former club for QPR centre-back Rio Ferdinand.
Angel di Maria put the hosts ahead midway through the first half and Ander Herrera and Wayne Rooney made the points safe for the hosts with goals before the half-time interval.
Juan Mata added a fourth early in the second half to mark Radamel Falcao's debut for United with a comfortable victory and leave QPR with just three points from their first four Premier League games.
And Redknapp conceded that he had envisaged a disappointing afternoon for his side.
He said: "A few minutes before half-time we were 1-0 down and still bang in the game. To come in at 3-0 down, when our keeper has not had a shot to save... If you have ever played in a team 3-0 down at half-time it is very difficult. You have to make sure you don't get beat by six or seven.
"[It was] a difficult day, I expected it and we got one."
The former Tottenham boss also revealed that midfielders Leroy Fer and Sandro were struggling with fitness issues.
Despite the reverse, Redknapp believes QPR have what it takes to avoid an immediate return to the Championship.
"Leroy Fer is not fit, Sandro got a bit of cramp where he's not played many games," Redknapp added.
"I still feel confident we will be OK."
