Athletic open their Europa League campaign tonight against the Giallorossi.
Williams said: "I'm super excited, we're all happy with a great desire to do well. For us it's a new experience, for others who have been lucky enough to play in European competitions it's a welcome return but we all want to do well."
Williams was also asked about the prospect of being a one club man.
He said: "Spending your whole career at the club would be a great source of pride, as far as my brother is concerned he should be the one to answer this question: what I can tell you is that we want to play for as long as possible with the Athletic shirt, we have a lot of fun playing together, we have also realized the dream of playing in Europe together.
"We will play against a very physical team with a lot of quality both in defense and attack. We have seen videos of the team with the new coach, it is a team that defends very well with man-marking, but we also have our strengths and I expect a quality match."