The superstar striker has already served a six-year stint at Old Trafford, but he transferred to Real Madrid in a world record £80m deal in 2009.
After another successful six seasons in Bernabeu, the Red Devils are hoping to secure his return in a blockbuster transfer this summer.
Man United have already made significant additions such as Memphis Depay and Bastian Schweinsteiger.
Portuguese international Ronaldo is currently under contract with Real Madrid until 2018.
Prying him away may be a bigger challenge than coming up with enough money to convince Real Madrid to sell the world-class striker.
Ronaldo scored a whopping 61 goals for Real Madrid last season, and the legendary striker has come up with 313 goals despite making only 300 appearances.
While he may not be as threatening in the Premier League at 30 years old, Ronaldo would instantly make Man United even more dangerous Premier League title contenders.
The Red Devils have already made progress by agreeing to sell midfielder Angel di Maria to Paris Saint-Germain.