The 29-year-old was surprisingly sold by the Blues to their Premier League rivals in a £40m deal, two months after lifting the top-flight crown at Stamford Bridge.
Matic has slotted in seamlessly at United, being used from the off in all 27 league games and eight times in other competitions, leaving Conte feeling regretful at the decision to sell the Serb.
"About this player I can talk only really well because he played with us last season," he told the assembled press ahead of Sunday's trip to Old Trafford. "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. 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."
Matic spent five years in West London across two separate spells, making 123 league appearances during that time.