The 31-year-old only moved to the Mestalla at the beginning of the season from Real Zaragoza, but has netted just three times during an unsuccessful campaign.
West Bromwich Albion's Nicolas Anelka has also been linked with a move to Lazio, but Reja has revealed that the Frenchman was "a red herring".
"The deal for Helder Postiga is going to be closed, he is a player we have been following for one week," Reja told reporters at a press conference.
"The name of Anelka was a red herring. I wanted a striker of international profile and Postiga is a complete player - I am very satisfied if he arrives."
Postiga has played for the likes of Porto, Tottenham Hotspur and Sporting Lisbon during a well-travelled career.