The banner, which read “Believe Kathryn Mayorga,” referred to allegations of rape made against the Portuguese international footballer in 2009.
Feminist campaign group Level Up claimed responsibility for the stunt and said a “culture of silence” has been created around Ronaldo which must end.
Ms Mayorga has claimed the Premier League superstar assaulted her in a hotel suite bedroom after she met him in a nightclub in Las Vegas.
Ronaldo maintains the sex was consensual.
The banner appeared around five minutes into Manchester United’s game against Newcastle on Saturday, in which Ronaldo was part of the starting line-up.
The striker received loud cheers from fans as he took to the pitch before the game, in which he scored two goals in a 4-1 victory.
Janey Starling, co-director of Level Up, said: “Manchester United has welcomed Cristiano like a hero, and created a culture of silence about the rape allegations made against him.
In 2019 lawyers representing Ronaldo won a US courtroom bid to block Ms Mayorga from digging into the validity of a confidentiality agreement made between them in 2010 in which she was paid 375,000 dollars.
Ms Mayorga, from Nevada, gave consent through her lawyers in 2018 to be identified.