Saints looked shell-shocked having started the brighter and were fortunate to go in at the break level, with Gaston Ramirez following up a Julio Cesar fumble.
Saints manager Mauricio Pochettino also celebrating his birthday sent his side out with renewed vigour but they were caught napping.
Nearly all the pre-match build-up surrounded Redknapp's first return to Southampton, who were relegated eight years ago under his leadership before he then rejoined bitter rivals Portsmouth.
"You're going down with the Redknapp" was one of the few polite chants directed at the QPR boss.
Harry Redknapp will be in the stands as Rangers take on Manchester United at Old Trafford this afternoon. Redknapp told the club's official website: "I'm delighted to be back in the managerial hot-seat at QPR.
"There's no doubt there's a lot of ability in the squad, but for one reason or another things haven't gone as well as everyone expected since the start of the season.
"I can't wait to get to work and get the results necessary to kick-start our season."
Redknapp, who left Tottenham at the end of last season, takes over a side four points adrift at the foot of the table and without a win from their first 12 league games.
QPR chairman Tony Fernandes said: "Harry was our number one target, the unanimous choice of the board and we're delighted we've been able to attract him to the club."
Fernandes continued: "He has a proven track record in the Premier League and has all the characteristics we were looking for to get us out of the current situation we find ourselves in.
"His man-management skills are second to none and we are confident he is the right man to lead us away from the bottom of the table."
A announcement regarding Redknapp's backroom team will be made in due course.
Harry Redknapp has today been named as the new manager of QPR on a two-and-a-half year deal.
The former Saints, Portsmouth and AFC Bournemouth boss replaces Mark Hughes at the London club.
QPR are currently bottom of the Premier League.
Harry redknapp is being widely tipped to become the new manager at Queens Park Rangers, after the Premier League side sacked Mark Hughes earlier today.
The former Portsmouth manager, who lives in Poole, was sacked himself by Tottenham Hotspur in a surprise move in the summer.
Hughes has undoubtedly been under more pressure because of the availability of Redknapp.
Rangers fans have been calling for Redknapp to step in and it would be a major surprise if a man with so much Premier League experience did not head owner Tony Fernandes' list.
Nicky Shorey scored Reading's second goal in the Capital One Cup match at QPR. The Royals went through 3-2 after a dramatic finish.
Reading's hero was Pavel Pogrebnyak as the Royals knocked QPR out of the Capital One Cup. Pogrebnyak scored a late winner to stun the London side. Reading twice came back from a goal down. At the end Pogrebnyak even missed a penalty.