Hasselbank led the R's to 12th in this season's Championship.
At the time, the west Londoners were six months out of the Premier League and still harboured promotion aspirations.
However, Hasselbaink's men ended the campaign the campaign 14 points off the play-offs in 12th place, and will be joined by his former side in next season's Championship.
The Dutchman has already begun preparations for next season, releasing seven players from his squad at the end of their contracts, though with Newcastle United, Aston Villa and Norwich City all coming down, securing promotion next season looks a difficult ask.
However, on his decision to move to Loftus Road, Hasselbaink is quoted as saying by Get West London: "I thought at the time it was the right decision for me to make and would I do it again? Yes I would."
Get West London reported earlier this month that Hasselbaink, who signed Burton forward Abdenasser El Khayati in January, could return to his former side for left-back Damien McCrory.