The Spain worldwide was the Premier League champions' top scorer last season with 20 goals in the top flight and is now keen to rejoin Atletico Madrid.
Morata was signed as a replacement in a £70 million deal from Real Madrid after the club missed out on first-choice target Romelu Lukaku to Manchester United, and while Chelsea winger Willian is happy to have the 24-year-old at the club, he is sad to see his friend Costa leave.
Last week, Costa's lawyer Ricardo Cardoso threatened to utilise "all possible legal mechanisms" against Chelsea and submit a formal transfer request in response to what he referred to as the "discriminatory behaviour" of head coach Antonio Conte towards his client, who was informed by text earlier this summer that his career at Stamford Bridge was over.
"Chelsea are weaker than last season - no Diego Costa, no Matic - and I'm concerned for them".
"It won't be easy to repeat an incredible season like past year", he said. He is a great friend who is unfortunately no longer with us.
Chelsea have made four signings - Alvaro Morata, Tiemoue Bakayoko, Antonio Rudiger and Willy Caballero - this summer, while allowing Matic, Nathan Ake, Asmir Begovic, Nathaniel Chalobah, John Terry, Kurt Zouma and Ruben Loftus-Cheek to leave either permanently or on loan. He will need to get used to the Premier League but then he will perform. "I like him and I hope it works out". "He's playing every game from the start [in preseason] and it's very important for him to play well". The transfer market is not finished. But for sure we want to try to fight, to fight until the end, to try to retain the title. Me and the other players are here to give all the support to our new players, for they can earn at Chelsea what they have been doing in their previous clubs.
"Alvaro is a really good player, he's still young despite having a lot of experience", he said.
"I think this is a great possibility for Michy", Conte said in a press conference on Friday.