Veteran tactician Reja called time on his second stint in the Stadio Olimpico dugout on Thursday night and former Bologna and Parma boss Pioli was promptly installed as his successor on a one-year contract.
Lazio finished a distant ninth in Serie A last season and Pioli, 48, is fiercely determined to steer the 2000 Scudetto winners back into a European competition as soon as possible.
He told the Gazzetta dello Sport: "My staff and I will give our all to make sure next season is an extremely positive one. We are here to do big things and we have to return Lazio to where they belong.
"The club offered me a two-year deal but I instead signed a one-year contract, which will automatically be renewed if we get back into Europe.
"I didn't come to Lazio in search of a long contract but instead to work with a competitive team, to play good football and to achieve some great results.
"There will be changes, some key players are yet to renew and others have left, so I will have to find some players to improve the team."
Pioli had been out of work since leaving Bologna in January - the Rossoblu were relegated four months later - and represented a surprise appointment to the Lazio job for some critics in the Italian media.
But Biancocelesti president Claudio Lotito has staunchly defended his decision to hire the former Juventus defender.
"My hiring of Stefano Pioli is a deliberate, well-thought-out choice and one based on the professionalism and morality of the individual as well as his technical qualities," said Lotito.
"Pioli knows very well how to work in difficult circumstances and he has shown enthusiasm for this challenge and wants to prove his worth.
"People must be judged after they have done a job and not before they have started. He will have to prove himself on the field of play.
"My approach was the right one."