Real Madrid were outplayed at their own backyard by pre-match underdogs Barcelona, culminating in a 4-0 victory for the visitors.
A brace from Pierre-Emerick Aubameyang and a goal each from Ronald Araujo and Ferran Torres cut Madrid's lead at the top of the table to nine points.
Los Blancos were missing top scorer Karim Benzema due to injury and Ancelotti tried to get creative by deploying Luka Modric in a false nine position which backfired spectacularly.
Ancelotti's substitutions after halftime had little effect as Barcelona increased their two-goal lead from the first half to four within six second half minutes.
Guti was not impressed with what he witnessed from the former Chelsea, AC Milan and Everton manager, and has blasted his tactics on the day.
"I'm Ancelotti and I look at the bench and I have Hazard, [Luka] Jovic, Isco... And you're telling me that I can't sub any of them in?" the former Madrid midfielder said when analysing the game on El Chiringuito.
"What's going on? I am getting lost. I watch the game against Barcelona and I see the Real Madrid bench and I find that in the second half the first change from Madrid is Mariano... How can this be?
"Aren't Bale and Hazard there? Madrid's first offensive change Mariano [Diaz], who hasn't played this year?"
Barcelona are confident of mounting a late title charge after the momentous victory as they are 12 points behind the leaders, having played a game less, with nine rounds of matches left.
