Coach Totchtawan Sripan's latest stint once again to failed to produce the desired results as True Bangkok United fell 1-0 at lowly Rayong on Saturday night.
Bangkok United's failure to record a win in their past three outings has sent the capital side tumbling down to the bottom half of the Thai League 1 table.
After a faltering start, Bangkok United had hired former league-winning coach Totchtawan last month in the hope of arresting the slide.
An impressive performance by Rayong, ranked No.15 in the 16-team top flight, was capped by a 68th-minute strike by Sirisak Foofung which proved enough to give the hosts three points from the game.
Both teams were reduced to 10 men when Suwat Junboonpha of Rayong and Manuel Tom Bihr of Bangkok United were sent off in the last minute of the match.
Meanwhile, former champions SCG Muang Thong United regained form, thrashing visiting Suphanburi 3-1 to end a four-game winless run in the league.
Derley and Sardor Mirzaev were the star performers for the Kirins as they combined to give the hosts a massive three-goal advantage by the break with Mirzaev claiming a brace and Derley adding one to the tally.
Suphanburi's problems were compounded one minute before the break when Pandecha Ngoenprasert was red-carded for a foul on Muang Thong's Wongsakorn Chaikulthewin.
However, the visitors did well to pull one back in the 82nd minute when Patrick Reichelt found the net for Suphanburi.
Samut Prakan City enjoyed some goal-scoring practice as they fired four unanswered goals past hosts Trat to grab sixth spot in the league.
Also last night, Nakhon Ratchasima pulled themselves out of the three-team relegation zone with an impressive 4-0 home victory over Police Tero.