Roma won the first-leg of their round 16 tie 2-1 at the Olimpico, with Eldor Shomurudov hitting the winner for the Giallorossi.
Before then, Nico Williams had put Athletic ahead before the hosts drew level through Angelino. Athletic then had Yeray Alvarez sent off, which handed Roma the advantage before Shomurudov's strike.
“The idea was to have many options before we started the match, then prepare for potential good and bad situations,” Ranieri told Sky Italia.
“It was a very difficult match, it will be even tougher on their home turf, but we also know that we are tough to beat and we fight for every ball. This is half-time, it remains 50-50."
Ranieri also said, “I think putting your faith in players is the most important thing a coach can do. Everyone makes mistakes, if you start listing them, then players lose confidence in themselves. I point out where they must improve, but also give them freedom to attack and cause problems for the opposition.
"These lads are splendid, they follow me in everything and it is a great joy for a coach to be in that situation.”
Ranieri also joked about the threat of the Williams brothers: “We need to keep going along this path, but we’ve got to make fewer mistakes with our passing and also move the ball around faster. It was tough today, as the two Williams players are so fast they may as well be on motorbikes.
“We have got the basics sorted out now, so it is time to work on those little details that can make a good team into a great one.”