Real Madrid manager Carlo Ancelotti said they must improve their defending at set pieces after conceding twice at corner kicks in Tuesday's 3-2 Champions League defeat by RB Leipzig.
Josko Gvardiol and Christopher Nkunku put Leipzig 2-0 up after 18 minutes in Germany after Madrid failed to deal with corners, as the Spaniards suffered their first defeat of the season.
"We weren't paying attention to the ball and then we conceded two goals, which we're usually much more convincing at doing," Ancelotti told reporters.
"I haven't seen a lack of intensity or attitude. Sooner or later a defeat had to come, I've said it before. It is a defeat that hurts, because all defeats hurt, but some less than others, because we still have another chance to finish first."
Madrid, who have already qualified for the last 16, top Group F on 10 points, one ahead of Leipzig with Shakhtar Donetsk on six. Celtic are bottom with two points.
Ancelotti said the loss to Leipzig would allow Madrid to identify their weaknesses.
"With a defeat you learn more than with 10 wins in a row," he added. "Because we are going to focus more on what has not gone well, which has mainly followed defensive set pieces, where we have conceded two goals."
LaLiga leaders Madrid next host Girona on Sunday.