The Hoops were beaten by a Lasse Schone goal early in the second half in their fourth fixture of the group stage but it could have been a different story if they were more efficient in front of goal.
In the end, however, Celtic were not competent enough in the final third to challenge the Dutch giants which left Lennon to rue the loss that forced them to the bottom of the Group H table.
"I thought we were poor first half, I thought we were a lot better second half," he told Sky Sports.
"The goal came at a time when we were just starting to get a foothold in the game. That's a poor goal from our point of view.
"But we had chances second half, particularly the two v one with Jamesy (Forrest) breaking - you're thinking it was a great opportunity and that might have just changed the course of the game.
"But I thought we were second best first half and some players will have regrets about their performance tonight because they weren't anywhere near their best."
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