Manchester City returned to the top of the Premier League on goal difference after coming from behind to draw 1-1 with Southampton at the Etihad Stadium on Sunday.
Pep Guardiola's side, who began the season with 10 straight victories, have now gone five games without a win in all competitions, equalling the Spaniard's worst run as a manager.
"You can win 10 times and then you are not able to win five times," said Guardiola. "I have to discover the reason why and I am going to find that.
"We had problems creating our build-up. There is a difference between the first and second half. Unfortunately, we could not win."
It could have been even worse for City after they gifted Southampton the lead when a wayward pass from John Stones put Nathan Redmond in on goal after 27 minutes.
Moments later Stones thought he had atoned by bundling the ball into the net but his goal was disallowed for a Sergio Aguero offside.
Guardiola reacted to the poor first half by replacing De Bruyne with Kelechi Iheanacho, who made an immediate impact by scoring from close range after Fernandinho found Leroy Sane with an exquisite crossfield pass.
Saints remained a threat on the break and Claudio Bravo saved well from Charlie Austin on 74 minutes while Aguero went close at the other end, where Raheem Sterling was a constant threat as City pushed for the elusive winner.