Neil Warnock called Simon Hooper's decision to refuse Cardiff a first-half penalty against Watford "bizarre" and claimed that the Wiltshire referee had owned up to making a mistake after the game.
Cardiff's hopes of avoiding Premier League relegation suffered a blow in a damaging 5-1 home defeat to in-form Watford, but the Hornets were only leading 1-0 when Josh Murphy was felled by Daryl Janmaat three minutes before half-time.
"When you play the game and you're in football, you know how goals change games," Cardiff manager Warnock said.
"There's no excuse for getting battered five. They're a good side but you can have confidence when you're two or three up, knock it around.
"Going in 1-1 with our tails up and the crowd behind us it's a different ball game.
"I still don't understand, for whatever reason, that it's not given. It's bizarre.
"He (Hooper) is in a great position and unfortunately none of his linesman could help him. It's the most straightforward one you'll get."
Warnock said the Video Assistant Referee system, which comes into force in the Premier League next season, would have reversed Hooper's decision "in 30 seconds".
"He said he's seen it back on video and he's made a mistake," Warnock added.
"What do you say to that? It doesn't help me when you get battered five.
"VAR would have solved that in 30 seconds, everybody on the pitch knew it was a penalty.
"Troy (Deeney) said it was a nailed-on penalty, so too (Adrian) Mariappa and all their staff on the touchline.
"I like Simon as a referee and he's not done it on purpose, he's upset himself about the situation."
Cardiff remain one point and one place above the relegation zone ahead of the rest of the weekend's fixtures.
Watford owed their win to a hat-trick from Gerard Deulofeu – who became the first Hornets player to score a top-flight treble since Mark Falco in October 1986 – and a double from Deeney.
Victory moved them above Wolves and into seventh spot, and Watford have lost only once in the Premier League in 2019 as well as reaching the quarter-finals of the FA Cup.
Watford manager Javi Gracia said: "I am happy with the way we are playing.
"We are competing in all the games and with this ambition, I am sure we can achieve something different this season.
"We have 40 points now which is good, but we want to keep going in the league and FA Cup.
"I am very proud of all my players, they are showing their quality and commitment.
"The team is getting better and better, but we must try to improve."
