John Eustace's first-half header condemned Rangers to only their fifth defeat of the campaign and moved the Rams to within a point of them in third.
With Leicester 11 points clear at the top, it appears four or five teams are now scrapping for the one remaining automatic promotion place and Redknapp admits it will be tough getting over the line.
"I don't think people understand - I think I read in the programme that only eight teams have bounced back into the Premier League with automatic promotion at the first time of asking since something like 1993," said Redknapp.
"It's very difficult. There's no guarantees. We had to re-build this summer. We got rid of 18 players.
"People get carried away by things. Have a look at big clubs like Derby, Brighton, who got 27,000 last weekend, and the like. Leicester, Nottingham Forest - big clubs. Burnley are playing well.
"There'll be plenty of twists and turns between now and the end of the season. It's going to be so tight, but I'm confident we'll be right there at the end."
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