The Australian, who can play on the right of defence or midfield, has been training with the Eagles for the past week.
He is a free agent having left Sydney FC at the end of their season.
However, Warnock seems less than certain about handing the 21-year-old a contract.
"Ruben's not a bad player but we've got a fair few midfielders and I'm not sure if that area is a priority for us," he told the South London Press.
"He's due to spend next week training with us as well."