The 21-year-old player was suspended by the club on January 30, 2022 over allegations relating to a young woman after images and videos were posted online.
Greenwood was facing charges including attempted rape and assault until the Crown Prosecution Service announced six months ago that the case had been discontinued.
On Wednesday, Manchester United said that they were working “through the final stages” of their internal investigation into Greenwood, having completed the “fact-finding phase”.
Riley, 37, who is a co-presenter on Countdown, posted to the social media platform X, formally known as Twitter, to express her opinions on the matter.
She said: “I won’t be able to support United if Greenwood remains at the club.
“As an example, when it comes to VAWG (Violence Against Women And Girls), less than 1% of rapes reported to police lead to conviction.
“We’ve all seen and heard enough. Pretending this is ok would be a huge part of the problem.”
She added: “It would be devastating for my club to contribute to a culture that brushes this under the carpet and send a message to perpetrators everywhere that you can continue to act as you wish with no consequences.
“I really hope they do the right thing.”
A club statement about the investigation read: “Following the dropping of all charges against Mason Greenwood in February 2023, Manchester United has conducted a thorough investigation into the allegations made against him.
“This has drawn on extensive evidence and context not in the public domain, and we have heard from numerous people with direct involvement or knowledge of the case.
“Throughout this process, the welfare and perspective of the alleged victim has been central to the club’s inquiries, and we respect her right to lifelong anonymity.
“We also have responsibilities to Mason as an employee, as a young person who has been with the club since the age of seven, and as a new father with a partner.
“The fact-finding phase of our investigation is now complete, and we are in the final stages of making a decision on Mason’s future.”
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