Chelsea named veteran Spaniard Rafa Benitez interim first-team manager until the end of the season, the club announced on its website Wednesday.
The 52-year-old Spaniard is due to meet the players at the training ground in Cobham on Thursday.
Martinez will replace Roberto Di Matteo, who was fired Wednesday with the team on the brink of becoming the first defending champion eliminated from the group stage. Six months ago, Di Matteo had guided Chelsea to an unlikely Champions League title.
With the move, Russian owner Roman Abramovich completed his most controversial day yet at Chelsea, handing former Liverpool boss Benitez a contract until the end of the season, having seemingly been unable to convince Pep Guardiola to take the job.
Announcing that Benitez was an "interim" rather than permanent appointment, Chelsea said in a statement on the team website: "Chelsea Football Club can confirm Rafael Benitez has been appointed interim first-team manager until the end of the season.
"The owner and the board believe that, in Benitez, we have a manager with significant experience at the highest level of football, who can come in and immediately help deliver our objectives."
The length of Benitez's contract sparked speculation Wednesday about Abramovich's determination to land Guardiola once the former 41-year-old Barcelona boss completes his one-year sabbatical in New York.
On Wednesday, Guardiola's agent, Josep Maria Orobitg told the Brazilian website, www.lancenet.com.br: "Pep Guardiola will not decide anything before January. I was with him in New York 15 days ago and he told me that he will not decide before January or February which team he will coach or if he will sit on any bench next season.
"It doesn't matter what happens, before that period he will not take any decision with any club nor with any national team.
Orobitg added: "Only he will decide his future, he has the personality to do that. Now he is with his family, resting, reading, enjoying his hobbies, watching football...in January he will start to think about which team he will lead next season."
Di Matteo, who was hired on a permanent basis in June, was dismissed a day after a 3-0 loss to Juventus in Turin left Chelsea's title defense in tatters.
Chelsea also is struggling in the Premier League, having picked up just two points from its last four games to fall to third place.
"The team's recent performances and results have not been good enough and the owner and the board felt that a change was necessary now to keep the club moving in the right direction as we head into a vitally important part of the season," Chelsea said in a statement.
Chelsea was in disarray when Di Matteo, then an assistant coach, took over from the fired Andre Villas-Boas in March, initially on a temporary basis. The Italian brought together squabbling factions within the squad and led the team first to the FA Cup title and then the Champions League, beating Barcelona in the semifinals and then Bayern Munich in the final.
Di Matteo, with his typical coolness, was also praised for the way he handled a racism case involving John Terry that engulfed the club. Chelsea is in the middle of another racism incident, having accused a Premier League referee of abusing one of its players.
However, Chelsea owner Roman Abramovich has showed his ruthless edge following the team's first disappointing run of results this season. Di Matteo's successor would be the ninth manager under the Russian billionaire since he bought the club in 2003.
Chelsea is in third place in Group E behind Shakhtar Donetsk and Juventus with one match left. The Blues next face Danish club FC Nordsjaelland, but they could still be eliminated even if they win depending on the result of the other game.
In the Premier League, Chelsea had been leading for much of the season but now trails Manchester City and Manchester United.
"The club faces a difficult task ahead in qualifying for the knockout stages of the UEFA Champions League as well as maintaining a strong challenge for the top of the Premier League while competing in three other cup competitions. Our aim is to remain as competitive as possible and challenge strongly on all fronts," Chelsea said.
Chelsea's next game will be a home Premier League match against Manchester City on Sunday.
The 42-year-old Di Matteo is a Swiss-born Italian who played for Italy's national team and also served as a midfielder for Chelsea. After his retirement, he went into coaching and led both the MK Dons and West Bromwich Albion before joining Chelsea's backroom staff.
"We will never forget the huge contribution he has made to this club's history," Chelsea's statement said.
Benitez has been out of work since being fired by Inter in December 2010. Although experienced, the appointment of Benitez would raise eyebrows given his trademark defensive approach. Abramovich is widely known to prefer his team to show a more attacking style, demonstrating this by spending heavily on attacking midfielders Oscar, Eden Hazard and Victor Moses in the close season.
Information from Press Association was used in this report.