Chelsea sacked manager Frank Lampard on Monday after 18 months in charge, with former Paris Saint-Germain boss Thomas Tuchel set to take over at Stamford Bridge.
Lampard led Chelsea to a fourth-placed Premier League finish and the FA Cup final in his first season in charge.
However, he failed to match much greater expectations this season after the club spent a reported £220 million ($300 million) on new players in the summer transfer window.
A run of five defeats in their last eight Premier League matches has seen Chelsea slip to ninth, 11 points behind leaders Manchester United.
Sunday's 3-1 FA Cup fourth-round win against Championship side Luton was not enough to save the former England midfielder.
The club said it had been a "very difficult decision" to part ways with their all-time record goalscorer, who won three league titles and the Champions League during his 13-year career as a player at Stamford Bridge.
"Recent results and performances have not met the club's expectations, leaving the club mid-table without any clear path to sustained improvement," Chelsea said.
In a rare public statement, the club's Russian owner Roman Abramovich said: "This was a very difficult decision for the club, not least because I have an excellent personal relationship with Frank and I have the utmost respect for him.
"He is a man of great integrity and has the highest of work ethics. However, under current circumstances we believe it is best to change managers."
Chelsea are aiming to have Tuchel, who was abruptly fired by French champions PSG in December, on the bench in time for the visit of Wolves on Wednesday.
After serving a transfer ban in Lampard's first summer window, Chelsea splashed out on Kai Havertz, Ben Chilwell, Hakim Ziyech and Timo Werner, but that outlay has failed to translate into improved results.
Lampard's points-per-game average of 1.67 is the lowest for any Chelsea manager since Abramovich arrived at Stamford Bridge in 2003.
His departure marks the 14th managerial change since Abramovich's takeover, which transformed the London club into English and European heavyweights.
"He is an important icon of this great club and his status here remains undiminished," said the Russian. "He will always be warmly welcomed back at Stamford Bridge."
Brazil international Thiago Silva, who joined Chelsea on a free transfer in August, tweeted his thanks to Lampard, describing him as a "legend".
Lampard's first managerial posting was at second-tier Derby. In his one season in charge, they reached the Championship play-off final, where they lost to Aston Villa.
"I don't worry for Frank. I'm sure he'll go on to have a great managerial career as I feel he handled himself fantastically well in that job. It's a tough job and he's done well at it," said Lampard's former England teammate Gary Neville, who predicted Tuchel will also suffer from Abramovich's hire and fire culture.
"Tuchel will be exposed to exactly the same rules as Frank. We'll be talking about him being let go in the next 18 months to two years, I'm pretty sure of that."
The German only became available a month ago when he left PSG, less than five months after taking them to the Champions League final, which they lost to Bayern.
The 47-year-old won back-to-back league titles with PSG, but that was not enough to satisfy the club's Qatari owners, who replaced him with former Tottenham manager Mauricio Pochettino.
Prior to PSG, Tuchel impressed at Borussia Dortmund, having started his managerial career with Augsburg and then Mainz.