United, who were a man and a goal down when the Jamaica international fell to the floor amid a melee from a corner, went behind to Matej Vydra's opener and lost Jason Pearce to a straight red card for a foul on Almen Abdi.
In response to that, Warnock adopted the cavalier approach of making all three of his substitutions by the time the second half had started so, when Austin was carried off on a stretcher two minutes after the restart, his side had to play the remaining time with nine men.
They were duly run ragged, with Vydra scoring again alongside goals from Abdi, Mark Yeates, Sean Murray and Troy Deeney. Michael Tonge's penalty was Leeds' sole response.
"I'm not devastated about the game but I am about Rudy, he has broken is leg in two places. We had made the subs and I wasn't bothered, I thought the game of football was irrelevant today," he said.
Leeds have yet to officially confirm the nature of the former SK Brann player's injury, but Warnock added: "I was told it was a double fracture but he shouldn't have been in that position (covering for Pearce at centre-half).
"I wanted to try and win the game with 10 men, but everything is irrelevant tonight, it's the least of my worries when I think that we have lost Rudy for so long. When our lads knew about it, they were distraught.
"Millwall next week is what makes football intriguing. Thank goodness we have a week off. It's all hands on deck now, we will try and do it for Rudy."