Roma meet Athletic in the first-leg of their Europa League round of 16 tie tomorrow night.
Asked about Athletic star Nico Williams, Ranieri enthused: "When you play zonal, you try to close down certain areas of the pitch to stop them getting the ball. Athletic are first in one-on-one dribbling.
"They have a lot of quality up front and you have to be careful. The pitfalls are those of a team that is fourth in the league, well-built, they all have Basques or youth team players, so they have that incredible sense of belonging.
"It's a team that is riding the wings of enthusiasm. It will be difficult here, very difficult there, but it will be difficult for them too."
Ranieri was also asked about Paulo Dybala's current form with Roma.
He added, "I'll tell him what I always tell him, that he has to have fun. Champions like that need to be supported, get them into the game because they have a vision that others don't see, he has to have fun, do a couple of defensive things, but have fun."