Experienced coach Jadet Meelarp's return as interim boss failed to inspire Port as the Khlong Toey-based Thai League 1 giants fell to a 1-0 loss at struggling PT Prachuap on Saturday night.
There was also disappointment for hosts Bangkok United as they let 10-man BG Pathum United off the hook to settle for a 1-1 draw and stay two points behind their opponents in third place.
Prachuap's winner was the result of a penalty awarded early in the first half, but on a whole it was a lacklustre show by Port.
A handball inside the box by Port midfielder William Weidersjo was confirmed by the VAR and Willen Mota coolly converted the spot kick for the hosts in the 18th minute.
Port did have a few opportunities after conceding the goal but they weren't clinical enough in front of the goal.
It was the fourth straight loss suffered by Port in the league.
Rabbits' fightback
The Angels were hoping that Chananan Pombupha's goal in the 37th minute would prove enough, especially after the Rabbits lost their star striker Diogo Luis Santo to a red card for a foul on Tristan Do one minute into the first half injury time.
Sumanya Purisay grabbed the leveller for Pathum after 58 minutes.
Former champions Chiang Rai United were 1-0 winners over Khon Kaen United in an away battle.
Felipe Amorim curled home the ball shortly before half-time to seal three points for the Beetles.
On Friday night, hosts Chiangmai United settled for a 1-1 draw with Chonburi.
Bill Rosimar scored the opening goal for Chiangmai in the ninth minute, converting a penalty kick awarded after a handball by Chatmongkol Ruangtanaroj.
The Sharks equalised through Yoo Byung-Soo in the second half.
