Ronny Deila refused to blame the officials for Celtic's defeat to Dundee United - but told his sloppy players it's time to sharpen up.
Jackie McNamara's side halted the Hoops' run of eight successive league wins as they moved to within four points of the Scottish Premiership leaders.
Stefan Scepovic had a goal wrongly chalked off for offside late on as United held on for their first victory over the Glasgow giants since May 2012.
Deila refused to point the finger at referee Steven McLean or his assistants - but the Norwegian did reveal his anger at the way his team allowed Nadir Ciftci and Stuart Armstrong space to put the home side in front. Leigh Griffiths gave Celtic hope with a late strike but John Guidetti's recent lean spell in front of goal continued.
The on-loan Manchester City frontman is the top Parkhead scorer with 11 strikes but has now gone four games without netting.
Deila said: "Today it was two things that were most disappointing. We conceded two easy goals. Secondly, it's the fourth match now that we have created eight to 14 chances but when you score one goal that is way too little. We have to be more effective at putting the ball into the net.
"The disallowed goal is disappointing. But over a whole season it is even-Steven."
Ciftci punished Efe Ambrose's fresh-air swipe at a loose ball inside the Celtic box when he fired United ahead after four minutes.
The Turk then set up Armstrong's header 20 minutes into the second period when he wriggled past Emilio Izaguirre before standing up a fantastic cross to the back post.
Scepovic was onside when he prodded home from Izaguirre's chested pass but was hard done by as the linesman's flag went up. But the Serbian did well to play in Griffiths for Celtic's late score.
"I'm annoyed about the way we defended. It is easy to keep the ball, push them out - don't let them go in and open up the goal. We did that twice and then you see what happens," said Deila.
"Could Guidetti's place be under threat? Yeah. We have big competition in the team, we need all the strikers we have.
"John started very well and now he has had a little dip.
"All strikers go through periods like this. But we have to focus on what is going to get him out of this situation."