Ronaldo scored 118 goals in 292 appearances for United between 2003 and 2009 under former boss Sir Alex Ferguson.
He won three Premier League titles, the FA Cup, two League Cups and the Champions League before leaving for Real Madrid in a then world-record deal.
Speaking on Friday before news of Ronaldo’s return was confirmed, current United manager Ole Gunnar Solskjaer said: “I didn’t think Cristiano was going to leave Juventus. It has been speculation this morning and the last few days of course.
“We have always had good communication. I know Bruno [Fernandes] has been talking to him as well. He knows what we feel about him and if he was ever going to move away from Juventus he knows we are here.”
Solskjaer, who played alongside Ronaldo during the latter stages of his career at United, described his former team-mate as a “legend” of the club and the “greatest player of all time”.
“I was fortunate enough to play with him, I coached him when I got my job (as manager of United’s reserve team),” the Norwegian added.
“Such a tremendous human being so let’s see what happens with Cristiano. Everyone who has played with him I think has a soft spot for him.”