Greenwood has not played for United since January 2021 after being charged with attempted rape, engaging in controlling and coercive behaviour and assault.
The striker has continually denied all accusations against him.
All charges were dropped by the Crown Prosecution Service in February this year, and the club have since released a statement on the matter.
talkSPORT understands that staff have been told to prepare for the potential return of the 21-year-old, with the club to announce the results of their own internal investigation.
But while a decision on Greenwood's involvement with the Red Devils is yet to be announced, Countdown presenter Riley insists that she won't be a supporter if he does return to first team action.
MORE ON MASON GREENWOOD
JORDAN: Man United would not be deliberating Greenwood decision if he was not valuable
Manchester United open up on 'detailed and thorough' Mason Greenwood investigation
Riley re-posted a clip of a United fan giving her thoughts to The News Agents podcast where she said she would 'walk away from the club' if Greenwood returns.
"I'm with Em," Riley responded. "I won't be able to support United if Greenwood remains at the club.
"We've all seen and heard enough. Pretending this is okay would be a huge part of the problem.
"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.
Most read in Football
Liverpool want three midfielders, Olise snubs Chelsea, Walcott retires, Mitrovic to Saudi
Harry Kane announces new Skechers boot deal and pair could be yours for eye-watering sum
JORDAN: Man United would not be deliberating Greenwood decision if he was not valuable
Fulham eye Arsenal striker as replacement for Saudi-bound Mitrovic
"I really hope they do the right thing."