Spanish striker Costa, who netted 20 Premier League goals last season, is yet to find a new club but does not have a future at Stamford Bridge.
Antonio Conte informed the 28-year-old by text that he was free to leave the champions and Costa is said to be closing in on a return to Atletico Madrid.
Matic also helped the Blues storm to the title last term but was allowed to secure a £40million move to rivals Manchester United following Tiemoue Bakayoko’s arrival in west London.
The Serbia international won the league twice in three years at Stamford Bridge but Willian expects him to be happy at his new club.
‘Everyone knows about Diego Costa’s quality, the striker he is, and I am sad because he is leaving,’ Brazilian attacker Willian told Goal.
‘He has not yet go to any club, but Morata, who has arrived, has quality and knows how to score goals too.
‘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.
Diego Costa 2016/17 stats
‘Matic and Diego are two great players. I am sad to see them leave, but I expect Matic to be happy in his new club. But Chelsea have signed Bakayoko, who made a great season at Monaco, to reinforce our team.’
Costa is a fiery character on the pitch but Willian, who joined Chelsea in 2013, insists he will miss his ‘great friend’ and wishes him luck at his new club.
‘My relationship with Diego is very good. He has a way on the pitch, but he is a super good guy and very humble,’ the 29-year-old added.
‘He is a great friend who is unfortunately no longer with us. I only wish good things for him and all the luck in the world.
‘I have great friends in Chelsea, like Diego and David Luiz. We [Willian and Luiz] are neighbours and we are always together in and out the field.
‘Kenedy is also a great friend, and apart from the Brazilians, I have a lot of affinity with Hazard. I get along well with all the players. The atmosphere at Chelsea is very good.’
Chelsea, who lost the Community Shield to Arsenal last weekend, kick-start their title defence with a home fixture against Burnley on Saturday.