Aguero has been struggling with a knee injury and Kompany has been out since January after suffering a calf problem in the FA Cup fourth-round victory at Stoke.
"Sergio is in Argentina but he will come back on Sunday," said Mancini. "Vincent is not in our plans for tomorrow. But I think he will be ready for the Newcastle game, like Sergio."
Mancini tried to inject some humour into proceedings by sporting a red nose, in recognition of Red Nose Day. However, the City boss accepted Kompany's injury has proved rather more serious than initially believed.
"It is difficult," said Mancini. "Vinny is a really important player for us and now it is 45 days he has been out. It is a big problem."
It hardly helps City this weekend given they are facing Everton, who have proved a difficult opponent to overcome in recent times. The Toffees have won five of the last seven games between the pair, and drew at the Etihad Stadium earlier this season.
"Every game against Everton is very hard," said Mancini. "They are a top team who always play at 200 per cent. They have good players and a good manager."
Meanwhile, Mancini has rejected speculation that he will take over as manager of Zenit St Petersburg at the end of the season, joking: "It is cold here I think St Petersburg is (colder)? It is absolutely not true."
And Mancini also rejected the theory of Arsenal manager Arsene Wenger that a lack of English representation in the Champions League quarter-finals for the first time since 1996 was a massive "wake-up call" for the Premier League.
"Last year Chelsea won it and this year English football has three teams in the quarter-finals of the Europa League," said Mancini. "For three or four years English teams have won, or been to the final. For one year, other teams have done it. When I read this thing about English football being down I thought it was stupid."
- Sports & Recreation