The 66-year-old trainer believes that his two former clubs are the reigning champions' biggest rivals in the race for the Serie A titleLazio and Napoli have the quality to fight for the Italian league title this season.
Reja, who coached the Biancocelesti from 2010 until 2012, and the Partenopei from 2005 until 2009, sees Juventus as heavy favourites to win the Scudetto, but feels his two former teams could take advantage if the Turin giants slip up.
"Lazio and Napoli got disappointing results at the weekend, but I believe that both teams have what it takes to challenge for the Scudetto," Reja was quoted as saying by La Gazzetta dello Sport.
"Juventus are superior to any other team, but they also play Champions League football and that could affect them in Serie A.
"Lazio and Napoli are the two first teams to possibly take advantage of it if Juventus slip up.
"AC Milan are going through a transitional year. Fiorentina are a team that could surprise this season. They are a good team with a great trainer, who has all my respect."
Napoli currently sit second in Serie A, while Lazio occupy third spot.
Both sides lock horns on Wednesday in their fifth league game of the season.