Erik ten Hag was sacked as Manchester United manager following Sunday's 2-1 Premier League defeat against West Ham United, the club announced Monday.
Ten Hag, who had his contract extended in the summer, had just four wins from 13 games to start the new season.
Ruud van Nistelrooy will take over on an interim basis and is expected to be in charge for the Carabao Cup tie against Leicester City at Old Trafford on Wednesday.
Sources told ESPN that United bosses made the decision to sack Ten Hag after losing faith that the Dutchman would be able to turn things around after a poor start to the new campaign. After earning just 11 points from nine league games, there are already concerns that United are too far adrift to qualify for next season's Champions League.
Results and performances in the Premier League at the end of last season were also taken into account.
United finished eighth in the table last season and are languishing in 14th after defeat at the London Stadium on Sunday.
Ten Hag was kept on in the summer after leading United to a surprise 2-1 win over Manchester City in the FA Cup final.
The decision followed an extensive end-of-season review during which club bosses, including new co-owner Sir Jim Ratcliffe, held talks with a number of different candidates.
But despite conversations with potential replacements, Ratcliffe and his INEOS team decided to keep Ten Hag in his job and extend his contract until June 2026. Van Nistelrooy, who played for United between 2002 and 2006, arrived as part of a revamped backroom staff along with former Go Ahead Eagles manager Rene Hake.
Hake is set to stay at Old Trafford to support Van Nistelrooy in his caretaker role. Van Nistelrooy has a year of managerial experience with Dutch side PSV Eindhoven. He left his role in 2023 after leading PSV to success in the Dutch cup.
Sources told ESPN that United have already started to assess options for their next permanent manager. Thomas Tuchel was a leading candidate in the summer, but the former Bayern Munich boss has since agreed to take over as England manager.
Ratcliffe, INEOS director of sport Sir Dave Brailsford and sporting director Dan Ashworth are all admirers of former England manager Gareth Southgate, although he has said he does not want to return to coaching until next summer.