Leno said that the manner of win was unexpected, but insisted that they still need to keep winning if they want to qualify from the group.
"It was a good performance from us," he told Bundesliga.com. "We didn't expect to win 4-0; Donetsk didn't play the way they can, to be honest. We expected them to attack us more, but we did a good job and gave very little away.
"It was important that we improved our goal difference in the head-to-head, as Donetsk are our direct rivals to go through. You could see we enjoyed ourselves today and, after making it 2-0, we still wanted to attack.
"Sometimes we get criticised for dropping down a gear when we go in front, but we didn't do that this evening. In fact we even went looking for a fifth goal. It's great when the team rewards itself with such a great result."
Leverkusen are currently second in Group A.