Liverpool's summer capture of Belgian keeper Simon Mignolet from Sunderland meant that long-serving Reina, who arrived from Villarreal in 2005, was no longer a guaranteed first0choice at Anfield.
"Obviously I know the situation and I was surprised," he told reporters. "We were trying to sign Julio Cesar [of Queens Park Rangers] and it was a surprise for us when Pepe Reina was available.
"For us it is a big boost and it is really important because he is a great player so we are really pleased.
"I would say he has a lot of respect for the club and he was surprised by the decision and because nobody had told him. He has a great relationship with the fans and the club so I don't think he has any problem with them."
Reina and fellow new signing Gonzalo Higuain could feature for Napoli in their Emirates Cup match against Arsenal on Saturday.