The former Crystal Palace captain moved to City yesterday for £30m, signing a deal to 2031.
“It is clear Marc has been one of the best defenders in English football for quite some time now, so we are absolutely delighted to bring him to Manchester City,” Viana said.
“I feel we have signed a huge talent who will help us improve. He is only 25, but he has shown already he is a leader, a brilliant professional and someone desperate to improve.
“He is strong, has exceptional defensive qualities, is an intelligent reader of the game, and brings passion and energy every time he steps onto the pitch.
“I am so pleased it was us he chose to join.
“Marc is entering the prime years of his career. I am sure every City fan is excited to see how good he can be in a sky blue shirt.”