Dave Bowman accused the Dundee United players of being "soft touches" after the 3-0 defeat to Partick Thistle.
Goals from David Amoo, Mustapha Dumbuya and Stuart Bannigan gave Thistle their first victory of the season.
But Bowman, who became interim boss following the sacking of Jackie McNamara, says the players must grasp the gravity of the situation.
"People think we are a soft touch and that second half performance proved that we are," said Bowman.
"It's just not acceptable."
The result leaves the Tannadice side bottom of the table after a run of eight games without a win - and just five points from 10 league matches.
Bowman called for the managerial situation to be clarified as soon as possible to bring some stability to the club.
"Every time there was a 50/50 challenge, I knew that Partick would come out on top," the caretaker manager told BBC Scotland.
"People think it's a false position. It's not - and the players have to grasp that."
Asked if the managerial situation needed clarified soon, he said: "I think it does yes. Just for clarity for everybody."
It was a different mood in the Thistle camp, with their manager, Alan Archibald, summing up his main emotions.
"I think it's more relief than anything else," said Archibald.
"I'm absolutely delighted for the lads and for the fans who have waited a long time for this."