Buriram United moved into the final of the Thai League Cup after a 1-0 victory over Chiang Rai United on Wednesday night but their coach Masatada Ishii said the Thunder Castle need to plug their leaky defence.
Buriram are seeking a sweep of all three domestic titles and are just one win away from realising their dream thanks to an eighth-minute winner by Theerathon Bunmathan.
The northeastern giants have already won the Thai League 1 and FA Cup trophies this season.
Buriram will take on 2019 champions PT Prachuap in the title showdown on Sunday at BG Stadium in Pathum Thani with the kick-off scheduled for 6pm.
The Killer Wasps defeated Chonburi by the same margin with Prasit Janthum delivering the winner with a long-range effort in the 63rd minute.
The event for the 2020 season, which dragged longer than expected and ended last year due to a coronavirus pandemic, was cancelled.
Ishii acknowledged after the victory over the Beetles on Wednesday night that Buriram had "some problems in defence.
"Everything went according to the plan and I would like to thank every player for the good work.
"We are looking forward to becoming the triple champions this season, but we did struggle in the defence in this game. We need to find a fix quickly."
Buriram United, who have won the tournament five times, had a fine start when Theerathon's lob over the head of Chiang Rai goalkeeper sailed into the net early in the game.
Chiang Rai could have pulled level in the 73rd minute but Felipe Amorin's shot missed the target and an Akkrawin Sawasdee header went wide three minutes later.
