Cardiff City 2 Leeds United 0
Marcelo Bielsa bemoaned missed opportunities and costly mistakes as Leeds were beaten 2-0 by Cardiff on their return to Championship action.
Leeds - the favourites to return to the Premier League as champions of the second tier - had 69% possession in the Welsh capital but could not make the most of it.
And errors at the other end allowed Cardiff to score in either half through Junior Hoilett and substitute Robert Glatzel.
The defeat leaves Leeds in second, level on points with leaders West Brom, and manager Bielsa said: "We could have won, we should have won. We created enough chances to score the necessary goals.
"They needed our mistakes to score, but we know we should have been clearer to finish the attacks. We lacked something in the match in the final third.
"Mistakes are part of the game. When we make a mistake we have to resolve it as a team.
"Everything is still in our hands. It's the same situation. The opportunity we had today was important and we couldn't take advantage of it. Now we have eight matches left and we cannot think everything is set."
Leeds, with Northern Ireland ace Stuart Dallas in the side, were dominant in both halves.
It was against the run of play when Cardiff opened the scoring through Hoilett, who picked up the ball after Kalvin Phillips gave away possession. He drove forward and his shot beat keeper Illan Meslier.
Hoilett celebrated with a Cardiff shirt sporting the name of Peter Whittingham, the former Bluebirds ace who died in March.
Cardiff had their backs to the wall in the second half but Neil Harris' introduction of Glatzel and Nathaniel Mendez-Laing allowed them to double their lead.
Leeds captain Liam Cooper gave away possession to Mendez-Laing and he passed inside to Glatzel, who fired home.