Ronaldo has been linked with a return to Sporting this summer.
Asked about signing the former Sporting prospect, Viana told Sky Sports: "It's time for Manchester United to stabilise and create a good spirit at the club.
"Ronaldo is a player who scores 20/25 goals a season, for sure.
"I don't think it's possible now. But you never know. He can decide where he wants to go and we never know the future. I don't want to talk too much about it, because it's always different talking about Cristiano.
"And I think he still has one more year of contract. Let's see what he decides."