Buckingham had to leave the Kassam early in midweek after feeling unwell during Oxford’s 1-1 draw with Derby County.
United assistant manager Craig Short told the media post-match that Buckingham had been checked over by the club doctor and recommended to head home after feeling ill with half an hour to go in the game.
“I went a little bit light-headed on Tuesday,” Buckingham said.
“I chose to remove myself and sit a bit more back in the dugout in the second half. I didn’t want to make a scene if I had fallen over in the technical area.
“I managed to get through the game and decided to not hang around afterwards and got myself home and had a good night’s sleep.”