Lee Bullen slammed an "unacceptable" second half show as Ayr fell to defeat at Dens.
United had looked well in the contest at half time after Dipo Akinyemi cancelled out Ryan Sweeney's early opener.
But the Dark Blues eased to success after the break through goals from Alex Jakubiak and Ayr old boy, Luke McCowan, leaving Bullen fizzing.
He told the Ayrshire Post: "For long periods of the first half I was really happy with the football we played on a tricky surface.
"We deservedly got our equaliser and probably went on to shade that first half and there were shades of how we played at Hamilton on Saturday.
"But the second half performance was completely unacceptable.
"We came in at half time and gave them a bit of praise for certain aspects of the performance but stressed the need for improvement in some areas.
"But we came out and weren't aggressive enough and lost every individual battle in that second half and that was the difference.
"We had a foothold in the game at half time but to come out and lose your individual battles like we did to a man is not good enough.
"If you want to be successful in this league then sometimes you need to grind it out after such a positive first half.
"The biggest bonus we have is a quick game coming on Friday night against Cove to get this out the system.
"But it doesn't matter who you play in this league - whether it's Dundee or Cove - you need to go out and win your battles and that's what we'll need to do on Friday."
Ayr made just the one enforced change to last night's team with Alex Kirk replacing Frankie Musonda.
But after a bright start they fell behind when Dees skipper Sweeney rose to head home from a corner.
However, Ayr hauled themselves level when Akinyemi skinned the Dundee captain and fired home his 22nd of the season.
United had plenty of the ball in the first half and looked the better side for large spells as they promised much.
But it was all Dundee after the break and Jakubiak burrowed his way through on the edge of the box to restore the lead after a period of pressure from the hosts.
And they wrapped it up late on through man of the match McCowan.
