Ronny Deila accused his side of conceding "very stupid goals" as Celtic lost 2-1 away to Dundee United yesterday.
After eight successive wins in the SPL, the Hoops had the chance to go seven points clear but crashed to their third domestic defeat of the season.
Substitute Leigh Griffiths netted in the 86th minute but the homeside were already two up by then thanks to goals from Nadir Ciftci and Stuart Armstrong.
Efe Ambrose was turned inside out for Cifti's fifth minute opener while Armstrong was unmarked eight yards from goal to head United's second goal midway through the second half.
"Today we gave away very stupid goals, that's something that we need to do something about," Deila said after the match. "It's about defending as a team as well as individual things, its so important to show strikers out on the pitch and not give them angles, we didn't do that twice today and they scored."
Celtic's finishing wasn't much better with Anthony Stokes and John Guidetti replaced by Griffiths and Stefan Scepovic on the hour mark with Celtic trailing 1-0.
Deila rarely switches away from his 4-2-3-1 system but it was some old-fashioned forward link-up play that brought Celtic their goal when Griffiths raced on to a through ball by Scepovic in the 86th minute.
Griffiths almost rescued a point with a shot that came off the post in stoppage time but Deila made it plain that he feels his side should be converting much more of their chances.
"We haven't been effective enough in the last three or four games," the Celtic boss added. "We have created so many chances, if you keep on missing them you will lose points and that happened.
"We had energy in our play, we had a lot of chances in the first half and should have been leading. I think that we played quite well, especially in the first half, we just need to put the ball in the net but that's the most important thing in football."
The defeat leaves Celtic four points ahead of Aberdeen and Dundee United at the top of the table with the Hoops next in action on Saturday at home to Ross County.
Elsewhere in the SPL, former Sligo boss Ian Baraclough got off to a winning start with Motherwell winning 1-0 away to St Mirren to open up a six point lead over the bottom of the table Paisley side.