Bayern were knocked out of the competition following a 6-3 aggregate defeat to holders Real after Cristiano Ronaldo notched his 100th Champions League strike with a hat-trick.
But Ancelotti was furious with the decision to send Arturo Vidal off for a second yellow card with six minutes to go, before Ronaldo seemingly netted two off-side goals.
Robert Lewandowski and a Sergio Ramos own goal had given Bayern hope of progressing but the former Chelsea chief was left unimpressed with Kassai.
When asked about the referee’s performance, Ancelotti replied: “Worse than our performance.
“I know it’s football and it happens sometimes, but not this serious of a mistake. I believe we played very well. I thought we deserved more.
“The decisions penalised us a lot. The card given to Arturo [after 84 minutes] wasn’t a card. The two goals of Cristiano were offside. It’s clear we’re not happy with this.
“A quarter-final, you have to have a referee with… I don’t know… with more quality. Or have video. There are too many errors.
“Video replays are necessary, Zidane agreed that the ref’s performance was bad.
“The referee made a big mistake, we are in the quarter final of the Champions League we expect a referee who is concentrating and focussed.
“We know it is difficult to make a decision on the pitch, but honestly they were clear and big mistakes.”