West Ham sacked head coach Nuno Espirito Santo on Tuesday after a disappointing start to the Championship season.
The 52-year-old Portuguese manager oversaw the Hammers' relegation from the Premier League last season but stayed with the London club.
Many viewed them as favourites to win automatic promotion back to the top flight but they are currently eighth in the second-tier table, six points behind leaders Middlesbrough, after 13 games.
West Ham had a good run after a shaky start to the season but have lost their past two league games, against Southampton and West Brom.
Nuno, who previously managed Wolves, Tottenham and Nottingham Forest, was appointed in September last year but was unable to keep West Ham in the Premier League.
"West Ham United can confirm that men's first-team head coach Nuno Espirito Santo has left his position with immediate effect," the club said in a statement.
"Following a thorough review of the start to the EFL Championship season, the board of directors believe that a change is necessary at this time in order to ensure that the club's objectives and expectations remain on track."
Assistant head coach Rui Pedro Silva and goalkeeping coach Rui Barbosa have also left the club.
West Ham said the process of recruiting a new head coach was already under way.