Graziano Pellè's unfortunate own-goal in the 56th minute of an otherwise-even game settled the contest.
"It was a good match on a high level because Leverkusen is a strong team and they showed that to us today," Koeman told the club's official YouTube channel after the game.
"It was not easy to create a lot of opportunities, but I think we didn't deserve to lose. We were very unlucky with the 1-0 from a corner kick and we created some good opportunities in the first 45 minutes, but OK it was a good test.
"Sometimes we have to know when to play a little bit quicker because next week we won't get the time to play easy. We have one more week to prepare ourselves in a little stronger way than we were today.
"It (the goal) was difficult I think because Steven Davis touched that ball from the corner kick and it was unlucky there was nothing that Artur Boruc could do about that - it was an unlucky moment which made the decision of the match.
"It's not cost us any points today and we know that we have to be on a good level to beat these kind of opponents and they showed us that. It was a good test and we like to have a good test.
"The result is not the most important thing. We did not deserve to lose today because we did enough to win or minimum to play a draw."
For the big stories and the banter follow us on Twitter: @tribalfootball