Mark Schwarzer's own goal condemned the Cottagers to a frustrating 1-1 draw against the Dutch club in a tie which they dominated for the most part.
"I felt you can't do more than we did in the first half," he said.
"Everything went well. We pressed them at the right times and scored from that [period] as well. We played good football and the second half was probably different because we tried to force the issue.
"Then they had some space to hit us on the break and they did that a couple of times.
"That is the only thing. If you don't score goals easily enough then you still have to try to.
"Keeping our shape is a lesson we have learned for next time but the worry of course is four matches and three draws. I have to say all those games we have been the better team.
"It is the same story. Against Aston Villa we were the better team, against Blackburn the same and now.
"Of course I cannot blame anyone because they did well but we have had to put a lot of energy in.
"With a game against Manchester City coming up [on Sunday], that is the only disappointing thing."