Southampton were tipped to struggle this season after selling Luke Shaw, Dejan Lovren, Calum Chambers, Adam Lallana and Rickie Lambert, as well as manager Mauricio Pochettino.
However, Koeman's new-look side sit second in the Premier League and maintained their fine form by winning at Arsenal in the third round of the League Cup in midweek.
Former Netherlands defender Koeman is not getting ahead of himself after such an impressive start to his reign and ruled out their chances of securing a top-four finish.
"[QPR] had two difficult away games – Man United and Tottenham. Every game is tough, difficult," Koeman said.
"It isn't realistic [to qualify for the Champions League]. I remember one of the questions [before] starting the season, and that question was totally different.
"We've played only five games in the Premier League. We know it's a long season, it's a tough season."
Saturday's clash will see Harry Redknapp return to his former club and the QPR expects a hostile reception.
Redknapp assumed the reins at Southampton in 2004, just weeks after leaving arch-rivals Portsmouth, and was unable to keep the club in the top flight.
He then quit midway through the following campaign, before returning to Portsmouth.
On his last visit to St Mary’s, Redknapp celebrated QPR's victory with a passionate fist-pump – an reaction which heightened his unpopularity with the Southampton supporters.
"You always get a bit of grief there," the 67-year-old said. "It was my birthday when we went there and won. It was a good birthday present, so let's hope we can go there and get another win."
Redknapp knows achieving that goal will not be easy: "They lost all those players and everyone thought, 'That's the end of Southampton'.
"Ronald Koeman has done an amazing job there and they play some great football. He was a fantastic footballer and he seems to have that same calmness about him as when he played.
"I've been very surprised by just how well they've done – they’ve been amazing. They're playing with confidence and it was an excellent performance at Arsenal the other night."
Koeman also spoke of his respect for the experienced Redknapp.
"I think Harry Redknapp is a name, a successful manager [with] a lot of experience,” said the Southampton boss. "He's a manager who I respect and I think everybody has to respect."
Redknapp confirmed Joey Barton and Jordon Mutch will miss out due to hamstring injuries, while Sandro (knee) and Adel Taarabt (ankle) are doubts as the London club eye only their second Premier League win of the season.
Southampton midfielder James Ward-Prowse has been ruled out, but Koeman hopes to have defenders Maya Yoshida and Toby Alderweireld back at his disposal following injuries.
The former sweeper could field Sadio Mane, Dusan Tadic and Graziano Pelle in an attacking triumvirate after they impressed at the Emirates Stadium.
