The England midfielder requested to be allowed to leave on-loan this week and attracted enquiries from six major clubs from across Europe.
But transfer expert Fabrizio Romano is reporting United chiefs, including Ratcliffe, agreed to block Mainoo's departure.
Speaking on his YouTube channel, Romano said: "Kobbie asked to go on loan, he never asked to go on a permanent.
"Never been a possibility, either for Man United or for Kobbie Mainoo. But he did want to go on loan. It was a priority for Kobbie Mainoo to go on loan.
"Man Utd said no, they decided internally, with Sir Jim Ratcliffe involved, that they couldn't sell Kobbie Mainoo.
"This was a decision made with all the people involved: directors, CEO, coach and owner. 'We can't let Kobbie Mainoo go,' that was the position."