Former Liverpool goalkeeper Reina - on loan at Premier League outfit Villa on loan from Italian club AC Milan until the end of the season - played with Koulibaly at Napoli.
The 28-year-old Senegal international central defender has been on the books of the Serie A club since 2014 and is widely considered as one of the best players in his position in the world.
Reina told Tuttomercatoweb when asked about the defenders he has played behind during his career: “In a moment, Koulibaly was something extraordinary.
“Difficult to choose, there have been many strong defenders. In recent years, the Koulibaly-Albiol partnership was phenomenal when they played.”
Stats
Koulibaly has made 14 starts and one substitute appearance in Serie A, and has played six matches in the Champions League for Napoli so far this season.
During the 2018-19 campaign, the central defender scored two goals and provided two assists in 35 league matches, and played five times in the Europa League and six times in the Champions League.
Italian football is suspended at the moment, and it remains to be seen when the campaign gets back underway.
Linked away
Koulibaly has been linked with moves away from Napoli in the summer transfer window.
According to Le10 Sport, Premier League duo Liverpool and Newcastle United are interested in signing the defender this summer.