The 36-year-old Portuguese international is gearing up for his second spell with the Red Devils, and Souness believes his new team-mates will benefit a great deal by having Ronaldo back at the club.
Talking on Sky Sports, the Liverpool legend said: "That effect he has in that dressing room, you won't get that if you don't have senior pros and they've got the ultimate. He comes into the dressing room, training will be different, attitudes will be different.
"I don't know if he's vocal or not, but he will pull a face if someone doesn't do something professional. When it's coming from him, that's enough. He's got a seventh sense, he's on another planet and I see him making the difference for them. What an opportunity to be in the same dressing room... 'Who can argue I'm the best that's ever kicked a ball?'"
On the changes made by Ronaldo, Souness added: " He plays the width of the box, what he's got, when you look at a player of that age - athleticism has dipped, looking at him it won't have, but it will. I think he will be the difference for them, because they were dropping points at home, their away form was fantastic.
"With him in the team, they will get goals. What won't have left him is cleverness, anticipation and the technique to deliver. There has to be reservations, because of his age. There was a watershed moment in training when I knew it was over [playing career]."
Neville had previously claimed that United's owners only signed Ronaldo to "shut people up" following the criticism they received last season. "The Glazers have done what they normally do when they get put under pressure," he said.
"They make two or three signings and basically, it shuts people up. I think there's deep rooted issues at United. They couldn't let Ronaldo go to Manchester City. They've had it in the past with the likes of Sanchez when he was going to Man City and they stepped in. They had to step in at this point, whether they wanted to or not is another matter.
"I made the predictions on Monday Night Football that it would finish Man City, Chelsea and Man United I'm not going to change that. I don't think the Ronaldo signing makes them a better team than Chelsea who won the Champions League winners last season and Man City were 20 points clear."