Robert Snodgrass was offloaded by Leeds United in 2012, with Norwich City buying the winger.
Warnock was in charge of Leeds for the 2012/13 campaign, and he felt that the Whites had a big chance going into the season.
However, just before the season was due to begin, Leeds sold Snodgrass to Norwich City.
And Warnock admits that the sale of the Scottish wide-man was the moment which cost Leeds their chance at promotion.
He said: “I was so disappointed when I was at Leeds. I thought I could have turned that club around. Pre season, I thought we’ve got a chance here. And a week before the season starts, Norwich came in for Snodgrass and Ken Bates said you’ve got to sell him £1 million.
“In the end we got £3 million, and he said you can have half of whatever we get. Unfortuantely, two days later Ken told me the bank had stopped him for spending any money.
“Snodgrass, I could have cried, he was the one chance we had. He came to see me and said I’ve got to go, and in the end he was right. That just about finished me off.
“But they’ve not done too well since I left.”
Warnock parted company with Leeds just before the end of the season, with the Yorkshire side sitting just above the relegation zone.
The experienced boss has gone on to prove his worth since departing Elland Road though, after guiding Cardiff City to promotion this past season.
Leeds, meanwhile, are still trying to return to the Premier League for the first time since 2004.
Leeds have continued to sell some of their key players over recent times, which has made building a promotion winning squad at Elland Road very difficult.
Snodgrass, on the other hand, knows exactly what it’s like to earn promotion to the Premier League, having done so Hull City and potentially Aston Villa, if Steve Bruce’s side win the play-off final this weekend.
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