Choi's side suffered a 6-2 aggregate defeat at the hands of Gamba Osaka in the last 16 of the competition, denying them a fourth quarter-final berth in five years.
Having lost the first leg 3-1, the K-League outfit were always going to be up against it going into Wednesday's second leg, and with Gamba scoring two first-half goals while Mauricio Molina missed a penalty for Seoul, the tie was all but over with 45 minutes still to play.
Seoul showed some fight despite the adversity, with Yun Ju-tae scoring twice in the second half, and Choi feels that this is one of the reasons his side should feel proud of their performance.
“This is the end of our ACL this year, but our players worked so hard and I’d like to praise them for that," he said.
“I tried to do various things, but things didn’t work out as much as I had wanted. The loss in the first leg and many goals given away turned the burden on our players.”
Choi admitted that his side were let down by their defensive frailties, with their ability to score goals not enough to overcome a leaky defence.
“Defence is important in any matches, and the team with high defending ability can advance in the tournament," he said.
“On top of that, there is a turning point to decide the winners and the losers in a big game like this, and you need a player who can show his high ability in such crucial moments.”