The former Barcelona defender claims his side could have easily left Signal Iduna Park with a point had they been a bit more clinical in front of goal
The Amsterdam giants appeared to be on their way to a hard-fought draw until Robert Lewandowski found the net from close range in the 87th minute, and their coach feels the side deserved better than a narrow loss.
"We had most of the possession. Of course they were dangerous at certain times, but so were we. We had the chances to score one, two or even three goals," De Boer said at a press conference.
"I said before the match that we had to play according to our style and that is what we did. Therefore, it is especially disappointing that we threw it away three minutes from time.
"We can be proud of the way we played, but we failed to take the lead. Against this type of team you can't afford to do that because you are then presented with the bill at the end.
"If you can blame us for something, it's that we were not cold-blooded enough to take the chances we got."
Ajax's next Champions League game is against Real Madrid on October 3.