April 26, 2017 0536 GMT
Bayern midfielder Arturo Vidal was sent off for two bookable offenses, while Real Madrid's Cristiano Ronaldo scored in extra time from an offside position as the Spanish holder won 6-3 on aggregate on Tuesday, BBC reported.
"We had thought of everything going into this game – except the referee," Ancelotti said.
"You can't decide a semifinal spot like this. It shouldn't happen at this level."
Ancelotti said Real manager Zinedine Zidane agreed with his assessment of Hungarian referee Kassai's "negative" performance at the Bernabeu.
"The referee probably wasn't up to the task," said the Italian, a former Real boss.
"I don't think Real Madrid have any influence on referees. The officials simply got it wrong. I had never been in favor of video technology, but I must admit it's necessary now."
Goalline technology – to determine whether the ball has crossed the line – was brought into the Premier League for the start of the 2013-14 season and the Bundesliga from 2015-16.
Video assistant referees (VARs) are being trialed worldwide, with FIFA president Gianni Infantino keen to employ the system at the 2018 World Cup in Russia.
VARs – which rule on key incidents concerning goals, red cards, mistaken identities and penalties – could be used in English football as early as August, with the EFL Cup first round touted as a possible initial testing bed for the technology.
Video has already been used at international level, with two wrong decisions corrected by VARs as Spain beat France in a friendly in Paris on March 29.
UEFA has not committed to the trials but in a statement said it was "constantly looking at ways in which it can improve the matches taking place in its competitions".
It added, "Regarding video assistant referees, there are currently trials taking place, so let's see the results and if or how the system can work while preserving the flow of the game."
'The referee killed it' – reaction
BBC 5 live pundit Pat Nevin said, "I don't want the referee to have spoiled the entirety of this game because it was a fantastic tie over two legs – but the referee killed it with the sending off.
"It was terrible, and then he gets the decision wrong for Ronaldo's goal.
"The referee will probably get as much chat as anyone else."
Bayern chairman Karl-Heinz Rummenigge said, "It was an extremely emotional spectacle, you can only compliment both teams. Our team has given everything, we must not forget that.
"It was great football, but unfortunately the referee gave some extreme decisions at our expense. The referee was of course the killer."
Zidane was a happy in the press conference after the game against Bayern Munich. His team went through the semifinals of the Champions League again mostly thanks to Ronaldo’s hat trick.
Praising Ronaldo’s performance he said, “He’s unique. We are happy with him and also Bernabéu’s supporters.”
Just few minutes before, Ronaldo said that he does not like to be booed by his own supporters.
Defending his side’s striker, Zidane said, “Perhaps he is not booed anymore, but this is Bernabéu stadium and it can happen from time to time, he knows it. He shows in the pitch what he has inside. There are few players who are there in the key moments and the supporters will thank Cristiano Ronaldo for everything he’s done here.”
The French manager also spoke about the game, highlighting Bayern Munich’s great performance: “It was very level, but at the end we achieved a good and deserved result. We can say we played against the best team. Today’s game was the hardest one, but our victory was well-deserved. We scored six goals and I am happy and proud of the great effort of my players. We had a cold mind, a lot of chances without scoring, so it’s normal to suffer in these kinds of games”.
Commenting on Ancelotti’s criticism leveled at the referee, Zidane sid, “Referees’ job is difficult, I never talk about them. Their second goal was also in offside […] After scoring six goals, I think we deserved to qualify. With effort, but we deserve it. There is no luck in football, you have to win and we’ve done it.”
On Sunday’s to El Clásico against Barcelona and the probability of Toni Kroos and Marcelo suffering minor injuries during his side’s quarterfinal fixture, Zidane said, “It’s normal that they have problems. But we have five days to recover. We are happy with the victory and are going to recover very quickly. The effort was huge and we are going to rest. This is while some players may have problems.”
Neuer crowns it all
Bayern Munich goalkeeper Manuel Neuer fractured his left foot during the match.
Neuer, 31, sustained the injury during Ronaldo's third goal in extra time.
The Germany keeper could be set to miss the rest of the season.
"Neuer will undergo a thorough examination on arrival in Munich prior to a decision on the appropriate treatment," said a club statement.