The 25-year-old will move on an emergency loan, but not until early next month.
He will move in a 93-day loan that will not be completed until their fixture with Birmingham City on October 3 to ensure the Sunderland man will be present over the entirety of the busy Christmas period, though it is believed that he will train with the club as of tomorrow.
The 25-year-old was brought to the Black Cats from Brighton by his former Seagulls boss Gus Poyet in 2014, but he failed to feature regularly under current boss Dick Advocaat and his last top flight appearance came in May.
However, despite falling out of favour at the Stadium of Light, there is no denying that Buckley is at worst a seasoned Championship star and realistically still has the potential to develop into a Premier League quality player as he approaches what should be his optimum years as a footballer.
The YEP say the Whites were eager to sign Buckley in the summer transfer window, but were reluctant to pay his reported £20,000 a week wages.
Leeds manager Uwe Rosler will be hoping that the wideman will demonstrate that quality at Elland Road over the next few months, ahead of what could be a possible permanent transfer later in the season.