The Serbia international completed a £21m move to the Blues from Benfica on Wednesday three years after leaving the west London side in the deal which brought David Luiz to the club.
Matic is not starting the game
However, the Chelsea boss gave rival manager David Moyes a small glimpse into his starting line-up for the crunch clash after revealing that Matic won’t start on Sunday.
“Matic is not starting the game,” Mourinho told his pre-match media conference.
“I feel that I hold respect to my players. He is my player now but I hold that respect to my other players.
“I don’t think it is the right message for them that a new player comes in immediately plays in the team.
“So I’m not playing Matic from the beginning.”
Mourinho added that Frank Lampard should be fit to face the Red Devils but defender Branislav Ivanovic is unlikely to feature after the Serb missed training this week.
“Lampard has been training all week with the team,” added Mourinho.
“Ivan has not had one single training session. Today he will train but I don’t believe he can play. Lampard I think can.”
MORE: Latest Man Utd news
MORE: Latest Arsenal news
MORE: The latest football news