The former Liverpool manager is surprised his old club were so willing to sanction the one-year loan move after the Serie A side had initially targeted QPR's Julio Cesar
Cesar was a transfer target for Benitez as he looked to sign an experienced goalkeeper before discovering that 30-year-old Reina was available following the arrival of Simon Mignolet at Anfield from Sunderland.
Benitez signed the Spain international as Liverpool manager in 2005 and jumped at the chance to be reunited with him.
"Obviously I know the situation and I was surprised," Benitez told reporters.
"We were trying to sign Julio Cesar 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.
"We knew he had a long contract so it would have been impossible and we were not even thinking about him.
"We were talking about Cesar and then we realised that he was available and that was perfect."
Reina's reputation in English football suffered last season as a result of uncharacteristic mistakes like the one that gifted Sergio Aguero a simple goal during Liverpool's 2-2 draw at Manchester City in February.
Benitez, though, has no doubts whatsoever over the calibre of the former Barcelona man.
He added: "He is still a fantastic keeper."