Chelsea manager Antonio Conte admits it is "unfortunate" that Nemanja Matic is now a Manchester United player.
Midfielder Matic played a key role in the Blues' Premier League title success last season before being allowed to complete a £40million summer move to rivals United.
"About this player I can talk only really well because he played with us last season," said Conte.
"I think he was one of the key players in winning the title. Now Matic, unfortunately, is playing for Manchester United. I wish for him the best.
"Not on Sunday, but in the future."
Matic was a regular for Chelsea last year, making 35 league appearances and also helping the London club to the FA Cup final.
Conte said last summer that selling the player was a "big loss" for the champions.
Asked why Matic was allowed to join a potential title rival, his former manager replied: "In the past we have already managed this topic.
"I don't want to come back and speak about the past because I think it's not important, not interesting for us."