The Reds fought back twice to tie the game at 2-2 just after half-time but were knocked out by two Robbie Fowler goals, including a penalty, to leave the Western Australian capital empty-handed.
The Reds' Dutch tactician was left to rue just how easily his second-placed side were opened by a Glory outfit who had saluted just once in their previous 16 outings.
"All of the goals we conceded were not necessary, they came from bad defending and there was always a mistake somewhere," Coolen said.
"We came back a few times and we could have scored a third goal and come back in the game, we played better in the second half and created a lot of chances but we made too many mistakes defensively, too many individual mistakes.
"So now we have to prepare ourselves for a couple of games that will be like finals and that are really important if we want to stay where we are now on the table."
Adelaide meet the Central Coast Mariners at Bluetongue Stadium on Boxing Day, December 26 - kick-off is at 5:00pm.