According to A Bola, Toro could bring in Viviano for just €1m, despite Sporting’s request for €2m.
The 33-year-old only joined the Portuguese club from Sampdoria last summer but never made an official appearance.
He spent the second half of 2018-19 on loan at SPAL, starting 17 games out of a possible 19 to help them preserve their Serie A status.
The former Italy international should play second fiddle to Salvatore Sirigu as the Granata prepare for a season that sees them challenge on three fronts.