Suspended Oxford manager Ian Atkins was refused entry to the Kassam Stadium on Monday - and learned minutes later that Graham Rix was taking temporary charge.
Atkins was suspended on Saturday after Bristol Rovers announced that he would take over as their boss next season.
Oxford chairman Firoz Kassam explained: "Ian arrived at the ground and I asked him to leave as I hadn't had time to speak to the Football League.
"He remains suspended until further notice," he told BBC Radio Oxford.
Atkins declined to comment on Rix's appointment, but he had already indicated that he does want to continue as United's manager in the short-term, and has not signed a contract with Bristol Rovers.
On Saturday, Kassam had explained that Atkins was suspended for breaching the terms of his contract.
"Ian has a contract after the end of this season. Between now and then I can extend it even if we don't get promoted, it's up to me," Kassam said.
"It's in my power to keep him for another year - I did explain that."
Rovers had announced on Friday that they had secured a deal to take Atkins to the Memorial Stadium on a two-year contract - but he would see out his contract at Oxford, which expires at the end of the season.
Pirates chairman Geoff Dunford explained: "We were advised that Ian's contract at Oxford expires at the end of this season, with no guarantee of an extension.
"Therefore the opportunity to manage at Bristol Rovers next season is a very attractive option for him."