The Dutch manager pointed out that his players wasted a lot of opportunities to secure the tie in the first leg itself, and they must prepare themselves for a tough fight in the second leg against the Croatian side next week.
"I thought we did very well in the first half - we played with high tempo and made chances to score," he told the club's website. "The second half was about controlling the game and going for a better result, but we didn't manage to do that and now next week will be difficult.
"In my opinion, we lost the opportunity to score more goals in the second half, because we dropped the tempo. We gave them the belief to dominate the game. We were dangerous in a few moments, but not like in the first half. That was about us - and I'm not happy about the second half.