The 22-year-old wing-back joined the Bull at the start of last season on loan and has settled in Turin.
Aina helped Walter Mazzarri's men keep 21 clean sheets in 32 appearances across all competitions, and played a key role as they finished the season in the seventh on the Serie A table with 63 points, scoring once and providing three assists.
On the back of his impressive performances, club president Urbano Cairo has stated that Toro will start the paperwork to sign the youngster.
“He’s done well, we’ll send Chelsea a letter stating that we will exercise our option to buy him,” Cairo told Sky Sport Italia.
Aina spent the 2017/18 season with English Championship side Hull City on loan and signed a new three-year contract with Chelsea shortly before he joined Torino.
The Super Eagles have been drawn in Group B along with Burundi, Guinea and Madagascar.