West Ham sacked manager Manuel Pellegrini hours after the club suffered a 2-1 defeat to Leicester City on Saturday.
That loss, their fourth in just five matches, leaves the Hammers sitting 17th in the Premier League, just one point clear of the relegation zone after 19 games.
Pellegrini joined West Ham in May 2018 with the former Manchester City manager guiding the club to a 10th-placed finish in his first season in charge.
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His second campaign has failed to get going, however, with the club losing its first league game 5-0 and winning only five of their first 19 matches in the Premier League.
To make matters worse, the Hammers were also knocked out of the Carabao Cup after losing 4-0 to League One outfit Oxford United in September.
Joint-West Ham chairman David Sullivan confirmed the decision to part ways with Pellegrini on Saturday night, stressing it was important to get the club "back on track".
“It is with great disappointment that we’ve had to make this decision," Sullivan said via a club statement.
"Manuel is a gentleman and it has been a real pleasure to work with someone of his calibre.
“However, it has become clear that that a change is required to get the club back on track in line with our ambitions this season. We felt it was necessary to act now in order to give the new manager as much time as possible to try and achieve that goal.”
Pellegrini's final game in charge saw West Ham fall 2-1 to the Foxes at home with Pablo Fornals' 45th-minute equaliser cancelled out by a Demarai Gray winner in the 56th minute.
Speaking after that defeat, Hammers captain Declan Rice admitted things needed to start turning around quickly with the club falling well short of expectations.
"People were talking about us being top-six contenders and now we're so far away," Rice said. "Fans are coming to the game pretty much not wanting to come. We need to pick up fast."
Overall, West Ham won 24 of their 64 matches under Pellegrini, losing 29 and drawing 11.
The club next faces Bournemouth on New Year's Day with the Hammers yet to confirm who will be in charge for that clash.