The Portugal international is set to join Samp over the coming days.
“I already wanted Adrien when I was at Leicester, but in the end the club only bought him the year after I'd left," Ranieri told Sky Sport Italia.
“I know him well, he's a midfielder with quality and character, but who can also defend and be a water-carrier when necessary. I was missing someone with his characteristics in the squad and he's excellent in the two phases, attacking and defending.
“On paper, this Samp team with Antonio Candreva, Keita Balde and Silva has been strengthened. I don't know if Gaston Ramirez is leaving, but if he does go and Valerio Verre remains, I'm happy with that."