The former Brighton winger is moving to United on a 93-day emergency loan and is expected to train with the club tomorrow but he will not sign in time for Sunday’s game at Middlesbrough.
Leeds intend to wait until their home fixture with Birmingham City on October 3 to ensure that Buckley is available up to and beyond the start of the January transfer window.
The rules of the Football League, which limit emergency loans to a maximum of 93 days, would otherwise see Buckley return to his parent club before the end of the Christmas schedule.
The 25-year-old, who has much experience of the Championship after spells with Watford as well as Brighton, is out of favour at Sunderland and was made available by the Wearside club during the recent transfer window.
Leeds enquired about him on more than one occasion and held talks about him in the final week of the summer window but their interest was dampened by Buckley’s wages, believed to be in excess of £20,000 a week.
United completed deals for Stuart Dallas from Brentford and Jordan Botaka from Excelsior in Holland but head coach Uwe Rosler remained keen on recruiting another wide player.