Reid missed all of last season with a knee injury, leaving Republic of Ireland boss Giovani Trappatoni to question whether he would ever play again.
But Rovers' manager Sam Allardyce has stuck by the 28-year-old and has allowed him to join the Loftus Road club on an emergency loan in a bid to rediscover his match fitness.
Reid said: "I've had a few injuries over the past couple of years and people may think I've gone off the radar a bit so this is an opportunity for me to come in and play a few games. I know what the Championship is all about from my time at Millwall. It's a great league to be involved in and hopefully I can bring some experience to the camp.
"There is clearly a good squad here. They're doing well in the league and hopefully I can add to that so we keep our place in the play-offs. If it goes well, perhaps my stay will be extended."