The club have been searching for alternative options to Luke Shaw all summer but failed to find a suitable solution before the deadline.
It is a frustrating situation for supporters who are concerned about the position, but as we at Flashscore reported recently, boss Michael Carrick remains an admirer of Shaw and hopes the club will be able to remain competitive in all competitions in the coming months.
There has been an acceptance that they could be left short of options if he suffers injury, and that is why they scoured the market for good short-term help.
Alejandro Balde and Jorge Salinas were viewed as two key targets. United even made another late check on Balde, 22, before the window closed while they held talks about 19-year-old Salinas, whose Racing Santander contract includes a €16 million (£13.7m) release fee.
In the late hours of the window, United were linked with a surprise move for a player from rivals Manchester City to solve the problem.
Ait-Nouri, 25, was the subject of a tentative late enquiry from United, but sources say there was never any possibility of such a move gaining traction.
The reason for United's late Ait-Nouri enquiry was that he was one of the few players they identified as genuinely being a level up from their current squad options, and felt he could be available.
Deals between the two Manchester clubs are extremely rare, and it is no surprise that City did not green light the approach.
United also considered David Raum as a signing but now have depth figures such as Patrick Dorgu, who is preferred as a left winger by Carrick, and Harry Amass.
The most likely back-up options for Shaw will be Noussair Mazraoui and Diogo Dalot, who Carrick believes are more than adequate in their ability to switch across the back-line from right-back to left-back.
United’s dream signing at left-back has been Newcastle United's £100m-rated Lewis Hall, and sources have hinted they may return for him in 2027, just as they waited to sign Carlos Baleba from Brighton.