Napoli have already won Serie A, Roma's Mourinho says before away tie
Napoli have 50 points after 19 games with reigning champions AC Milan second on 38 points and third-placed Lazio on 37 points.
Mourinho, whose side also have 37 points but are fifth on goal difference, said Napoli can expect a tough challenge from Roma when the teams face off in Sunday's clash in Naples.
"I say Napoli have already won the Scudetto, deservedly, so I congratulate them," Mourinho told reporters on Saturday.
"They are an excellent team with a great coach, sit 12 points clear with nobody who can frighten them. The Scudetto is theirs.
"If you ask me 'Will Roma get out of their way?', I say no, we go there trying to get a result. But that doesn't change the fact they have already won the Scudetto."
Mourinho said Nicolo Zaniolo (23) will stay at the club despite the winger publicly revealing his intention to leave.
Sky Sports reported that English club Bournemouth had offered Roma 30 million euros for a potential transfer but Zaniolo rejected it.
"He's been telling everyone he no longer wants to wear the Roma jersey, but unfortunately he seems to be staying," Mourinho said.
"I say 'unfortunately' because the player has been saying for a month that he wants to leave. If a player tells everyone he no longer wants to wear the Roma jersey, I have to say 'unfortunately'.
"The ideal situation is for the player to be happy. I said that I expected him to stay, but as things stand today, I have to add 'unfortunately' to that sentence."