Rhinos raced into a 10-0 lead after just nine minutes at Hull last night, but then the wheels came off.
Hull hit back at the end of the first quarter and then ran in four converted tries in 10 minutes before the break to go 30-10 ahead.
Leeds limited Hull to just four points in the second half, but that was little consolation as they slumped to their fifth defeat in six games.
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Furner said: “I thought the period in the first half from 18 minutes to half-time was probably what I’d describe as a car crash. Defensively we were not to the standards we need to be.
“There were some missed tackles that weren’t good enough and our discipline in that time wasn’t good enough.
“I am disappointed because the squad can be better than that.”
Furner added: “Put your shoulders in, make your tackles – that’s all you’ve got to do.
“We started well, then when things weren’t going our way we tried to solve it on our own instead of making our tackles and working together.”
Carl Ablett had been due to make his first appearance of the season after ankle surgery, but withdrew after the warm-up.
Furner admitted that was a blow, but insisted: “The players we selected there, I was confident they would be able to do a job.”
Harry Newman scored a try in his first appearance of the year and Furner described that as a “positive”.
Match report and pictures p36-37