Man Utd had planned to make a formal approach for Hall, but the move has not materialised, and there is now a growing sense that they will pull back from the idea.
Newcastle had no intention of selling Hall and would have placed a significant valuation on the defender. But this summer has already shown that they can struggle to hold on to their best talent when challenged, making it possible that the Red Devils could have tested the situation with an official offer.
Man Utd made initial enquiries through intermediaries and felt there was a good level of interest from Hall in making the move. Yet sources are now unconvinced that they will take the deal forward, because of Newcastle's stance.
One of Man Utd's clear objectives has been to ensure they do not overpay for new signings in this market and, as such, new transfer targets have begun to emerge.
Attention is turning to other options, with sources confident that Man Utd will still sign a new left-back before the transfer window closes.
RB Leipzig’s 28-year-old defender, David Raum, is in the picture and would add a good level of experience. He could be available for £34 million if Man Utd were to make an approach.
Club Brugge defender Joaquin Seys, 21, is more of an emerging talent also admired by the Red Devils' recruitment team and is valued at a similar price point. Racing Santander's Jorge Salinas, 19, has also been scouted.
Premier League-tested options could include Fulham's USA international Antonee Robinson, or West Ham United's El Hadji Malick Diouf.
Man Utd have Luke Shaw available for this season, while Patrick Dorgu is likely to be used as a left-sided attacker as well as a left-back. Noussair Mazraoui is an established player who could be used in the role if needed.
The transfer window has started to stutter for Man Utd, who are also now planning for life with Marcus Rashford back in the side. He will be competing for a position on the left flank if he stays, but could also be used at centre-forward.
The ideal plan this summer was for Man Utd to sign a third central midfielder, a left-back and a forward. While there remains confidence that they will bring in a new full-back, parts of their plan are certainly looking uncertain.