He can provide the width that United need
Manchester United were known for their attacking play and although Louis van Gaal did spend a lot in bringing Martial and Memphis, they still could do with a player who knows all about the club. Ronaldo still has got the goods in him and being a senior player, he could actually provide the team with a lot of leadership ability.
That being said, bringing him in would obviously cast a shadow on the leadership role of Wayne Rooney but Van Gaal has dealt with big egos earlier and Ronaldo coming back shouldn’t be a problem at all. The Real Madrid star is a legend at the club having scored 118 goals and winning three Premier League titles and a Champions League. He can play out wide or through the middle solving all the attacking problems that United have at the moment but would he be willing to move in January or in the summer that would be the main question.