In a letter to fans, Reina said that he had not been consulted by Liverpool about the move. His manager, Brendan Rodgers, has already signed Simon Mignolet from Sunderland as a replacement.
Reina's move to Italy was confirmed by the official Liverpool website this morning.
The goalkeeper will spend the 2013-14 campaign with the Italian side, who are managed by former Reds boss Rafael Benitez.
Reina moved to Anfield from Villarreal in the summer of 2005 and has since made 395 appearances for the club.
As one of Liverpool's longest-serving players, he announced on Twitter that he had written them a goodbye letter.
The comments were picked up by UK newspaper The Sun:
If I have one regret, it is the way that I am leaving.
It is only natural that I would be disappointed that the Liverpool management agreed to loan me to Napoli without telling me first, I thought that I deserved better than that even though I understand that difficult decisions have to be taken in football.
A lot has been made about me informing the club that if an offer came in from Barcelona that I would have liked them to consider it.
But I had also spoken to the club about the possibility of extending my contract if the offer was not made.
I told the manager that I wanted to play for Liverpool and that Barcelona would only become an option for me if the opportunity arrived, like the rumours said it would, as it would be a chance for me to go back home.
When it didn’t come I was happy to fight for my place so I was surprised that Liverpool decided it was in the club’s interests to send me to Napoli instead.
But Reina, despite being unhappy about how he left, is more than happy about his time at Liverpool. As Transfermarkt records, he won the FA Cup in 2006 and the League Cup in 2012 with the club. And he says that the Anfield crowd are what he will remember most:
I now understand how supporters can lift a team and inspire them to do special things.
I appreciate that there is something unique about Anfield and the atmosphere it creates.
I believe that anything is possible no matter what the situation is because at Liverpool there is no such thing as a lost cause. And I have been humbled by the fight for justice for the 96 which showed me that the Liverpool people will always fight for what they believe in.
I know that I have been blessed to be part of a club like that and I hope that in return I have always given my best and represented Liverpool in the right way.