The Colombian marksman's Sundowns contract will expire at the end of the current campaign - having decided against signing a new deal with club.
The Soweto giants Chiefs and Pirates are believed to be monitoring Castro's situation at the Chloorkop-based outfit.
Recent reports have indicated that the 28-year-old forward has fallen out with Sundowns head coach Pitso Mosimane.
Downs attackers Percy Tau and Sibusiso Vilakazi have been preferred ahead of the hard-working frontman.
Castro has made only two league appearances for the Brazilians this season and he was introduced in the second-half against Chippa United over weekend.
The former Cúcuta Deportivo striker could not help Sundowns avoid a defeat as the Chilli Boys won 1-0 in a Telkom Knockout Cup Last 16 match Sunday.
Castro, who joined the Tshwane giants prior to the 2015/16 season, will be free to sign a pre-contract January, 2018.
To date he has netted 10 goals in 39 league appearances for the Brazilians.