The Red Devils have bounced back from successive Premier League defeats by winning their last two domestic outings, most recently ending Watford's 100% start to the season on Saturday.
United will face Young Boys in their Group H opener on Wednesday night, and Mourinho believes that his side have struck a more effective balance within the team in recent weeks.
"I think we are improving. We have found a balance in the team, especially with the competitive attitude," Mourinho told reporters at his pre-match press conference.
"We are creating plenty of chances, we are changing how we attack opponents and protect ourselves in situations.
"Two difficult matches, both away, and we have six points. I am happy but I was happy after Tottenham - not with the result but with what I saw. We felt as a team that match was the beginning of our improvements."
United currently sit eighth in the Premier League table, six points behind leaders Chelsea and Liverpool.