The Uzbeks were beaten 3-1 in Tashkent in the first leg of their semi-final and travel to last season's K-League runners-up as outsiders to make it into next month's title match.
"Our chances of reaching the final are now slim but we will do our best in the second leg to recover," Kasimov told www.fifa.com.
"Although it will be a tough game at Ulsan, we must put the first-leg disappointment behind us. This is our last chance and we should do our best."
Bunyodkor can take encouragement from the fact they created a host of chances in the home leg, with Jasur Khasanov even missing a penalty, but their first task at the Ulsan Munsu Football Stadium is likely to be the containment of Ulsan's attacking duo Rafinha and Kim Shinwook.
The other last-four tie is more delicately poised with Al Ittihad leading Jeddah rivals Al Ahli 1-0 heading into the clash at the Prince Abdullah Al Faisal Stadium - also the venue for the first leg.
Al Ittihad boss Raul Caneda can welcome back defender Redha Tukar from suspension and will hope Naif Hazazi can continue his prolific form.
Hazazi scored the only goal in the first leg and now has eight in the competition.
Al Ahli boss Karel Jarolim is optimistic midfielder Yasser Al Fahmi will be fit after struggling with injury but both defender Kamel Al Mousa and midfielder Adulrahim Jezawi could miss out due to groin problems.