Tevez, 27, returned to the club's Carrington training ground on Monday after a potential move away from England failed to materialise.
And City coach David Platt confessed to The Sun that he is unlikely to be fit to face Swansea City in the opening match of the season, saying: "He's had three weeks off - I think it'll be tight."
The forward had repeatedly expressed his desire not to return to the Premier League side and will reportedly lose the club captaincy to central defender Vincent Kompany.
Aguero, 23, was brought to the Etihad Stadium in July for a reported £35 million from Atletico Madrid, likely with a view to replacing Tevez, but is still recovering from his spell with the Argentina national side at the Copa America.
And his new manager Roberto Mancini confirmed: “Sergio is not ready to play yet.”
The striker was an unused substitute when Manchester City lost 3-2 to Manchester United in Sunday’s Community Shield, while Tevez was not in the squad.
And now with both players unavailable for the Swansea match, Mancini will likely be forced to keep faith with the likes of Edin Dzeko and Mario Balotelli, who started at Wembley.