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.
Chelsea will be reunited with their former player at Old Trafford on Sunday and Conte expressed regret at the Serbia international's Stamford Bridge departure.
"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."
Fourth-placed Chelsea travel to the north west three points and two places behind their opponents.
They produced a morale-boosting performance in the midweek Champions League clash with LaLiga leaders Barcelona, although a mistake from defender Andreas Christensen resulted in the Spanish side snatching a precious away goal in a 1-1 draw.
Conte has backed 21-year-old Denmark international Christensen to bounce back from the error and believes he can be a future Chelsea captain.
"Personally I forgot this mistake," said Conte.
"If I had to talk about all the mistakes I made in my career, we'd have to be here for one day... it doesn't matter.
"Christensen is having a fantastic season and showing great maturity.
"Apart from the mistake, his performance was great, was wonderful.
"I have a lot of trust in Christensen. This player is a player for the present for Chelsea, and for Chelsea's future for 10 or 12 years.
"He can also become the captain of this team in the future."
