United are wary of Real's counter-attacking threat.
Carrick said: "We can't go gung-ho. I don't think we will and I don't think they will. It's hard to know, isn't it?
"The game over there was tight at times, but we could have come away with a 3-1 win and so could they. So it's about taking our chances.
"With the strike power we've got and the strike power they've got, both teams will have to defend well. You have to have the complete game in many respects."