Club captain Kompany has been out since suffering a calf injury during the FA Cup fourth-round victory against Stoke in January, and Mancini admits that losing him for this long has been a "big problem" for his team.
Aguero has been in Argentina undergoing treatment on a knee injury, and he has not featured since Feb. 24. His national side had hoped he could play in its upcoming World Cup qualifying games against Venezuela and Bolivia, but Mancini has revealed that he will return to Manchester.
"Sergio is in Argentina but he will come back on Sunday," Mancini told reporters. "Vincent is not in our plans for [Saturday]. But I think he will be ready for the Newcastle game, like Sergio.
"It is difficult. Vincent is a really important player for us and now it is 45 days he has been out. It is a big problem."
The Premier League champions face a tough trip to Goodison Park to face Everton, who Mancini says plays at "200 percent" at all times.
"Every game against Everton is very hard," Mancini said. "They are a top team who always play at 200 percent. They have good players and a good manager."