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 Redsâ€™ 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.
â€śAll of the goals we conceded were not necessary, they came from bad defending and there was always a mistake somewhere,â€ť Coolen said.
â€śWe 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.
â€śSo 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.â€ť
Adelaide meet the Central Coast Mariners at Bluetongue Stadium on Boxing Day, December 26 â€“ kick-off is at 5:00pm.