Romero has fallen down the pecking order at Old Trafford after the return of Dean Henderson but was priced out of a move away on deadline day because of United's insistence on a fee of around £10 million.
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The 33-year-old, a popular member of the squad, is set to return to Manchester after a break in Argentina but is likely to find no prospect of first team football until at least January while No.1 David De Gea and Henderson share duties in the Premier League, Champions League and Carabao Cup.
It comes just months after manager Ole Gunnar Solskjaer surprised the United dressing room by omitting Romero from the FA Cup semifinal against Chelsea and Europa League semifinal against Sevilla last season, despite picking the Argentina international for the majority of games in both competitions until that point.
Romero has also been left out of this Champions League squad after Solskjaer chose 37-year-old Lee Grant as the third goalkeeper in the list submitted to UEFA. It has led members of United's dressing room to express their sympathy with Romero's situation with many to feel he has been unfairly frozen out after five years of service.
Everton were keen to sign Romero on deadline day but were unwilling to match United's asking price and instead opted for Sweden international Robin Olsen on a season-long loan deal from AS Roma to provide competition for first-choice Jordan Pickford.
Romero has made 61 appearances since arriving from Sampdoria in 2015, keeping 39 clean sheets and conceding 27 goals. He has one year left on his contract, while United could also trigger an option to extend it until 2022.
His failure to secure a move on deadline day prompted his wife Eliana Guercio to post an emotional message on Instagram.
"It is a very sad day because he desired the chance to go to another club," she wrote. "He has worked hard every day for this, because he gives his all for United and I think that his chances of playing for United have gone down.
"This doesn't sound right to me. It doesn't appear honest or professional or serious that they have to let him go. I am very sad because of this."