The Italian tactician, normally a picture of sideline serenity, could only shake his head as his injury-ravaged defence struggled against Barcelona's relentless assault at a sold-out Montjuic Olympic Stadium.
Even with a hat-trick from Kylian Mbappe, Madrid were handicapped by the absence of their top defenders - Eder Militao, Antonio Rudiger, Dani Carvajal, Ferland Mendy, and David Alaba - due to injuries.
The Madrid defence struggled to contain Barcelona, who were spurred on by teenage sensation Lamine Yamal and Brazilian forward Raphinha, both of whom found the net.
"We made some glaring mistakes that cost us several goals," Ancelotti said.
"In high-level matches against top opponents, you need to be on top of your game. We played well in attack, but it's clear we can improve a great deal on defence."
After Madrid's two-goal lead, courtesy of Mbappe, Barcelona's Eric Garcia outjumped Madrid's defenders to score from a corner.
Along with Yamal's remarkable goal, two glaring errors from Madrid gifted Raphinha a double and sealed the game for Barcelona.
With Barcelona seven points clear at the top of the table, they can secure the LaLiga title as early as Wednesday if Real Madrid fail to win at Mallorca.
However, irrespective of Madrid's result, a Barcelona victory in their Thursday derby against Espanyol would guarantee the Catalans the championship.
"Everyone can make whatever assessment they want," Ancelotti stated when asked about last year's champions' disappointing season that is set to end without any silverware.
"We have to finish the season well and try to win the three remaining games."