"It's very difficult," Redknapp told the Hotspur magazine.
"You have to be so careful because if you open up too much you can get absolutely ripped to pieces.
"They have so many fantastic players and can hit you from so many places that if you think you can go and match them up and play an open game then you could be on the end of a difficult 90 minutes.
"And that goes for anyone who plays them. You respect but don't fear them because we also have good players for them to worry about.
"First and foremost you have to be aware of how they play in midfield. For instance, Paul Scholes or Michael Carrick will drop deep to pick the ball up off the back four, so you are looking at one of the strikers to get around that because if a midfielder does it, one of their front players will come into the hole he's just come out of.
"Then, of course, you have a problem that could end up with a 'one against one' at the back.
"You have to work hard against them and make sure you are set-up to stay in the game. We'll be doing exactly that.
"You can't just think 'okay, we'll play 4-4-2 and attack them all over the place' because they are possibly the best team in Europe. But we will be getting at them in all departments.
"You have to make sure you stay in the game and therefore there is no point going out there and starting gung-ho. You need to adopt a good shape, be solid, have a plan and know what you are doing against them.
"But we won't sit back. We will have a go.
"We have good attacking players and we hope to cause them problems as well."
"People said to me, 'it would be great if it was Derby' but I wanted to play United. It will be a special day."