The 30-year-old's future at Anfield has been in question ever since the arrival of Simon Mignolet from Sunderland last month in a deal believed to be around £9million.
Rodgers expressed concerns about the financial implications of having the two goalkeepers on the wage bill, and following a successful medical at the Stadio San Paolo earlier this week, a loan move has now been finalised.
"It was obviously a difficult decision," Rodgers told the club's official website. "From last season, really, as a manager I was led to believe that we were going to get an offer for Pepe that would probably see him leave the club.
"On the football side, we have to protect the team, and at that stage we started our search for another goalkeeper.
"As events transpired, that move (for Reina) didn't happen and it left us with two very good goalkeepers. Both would have been fighting for the No.1 position.
"It's decided he will go to Napoli and we've got two very good goalkeepers still at the football club (Mignolet and Brad Jones).
"As a senior goalkeeper, he needs to be playing. It's a very important year for him with the World Cup next year. It wouldn't have served any purpose, both for the club and him personally, if he'd been the one missing out."
Reina has spent eight years at Liverpool, having arrived from Villarreal in 2005.
The move to Serie A will see him link up with former boss Rafael Benitez, following his own switch to Naples at the end of May.
Despite clinching 25-year-old Mignolet on a long-term contract, Rodgers has, however, not completely closed the door on Reina's Liverpool future, adding "We'll review it after the loan finishes."
