The 34-year-old has been a free agent since his contract at Real Madrid expired at the end of June, but United sources say he simply does not fit the club's new recruitment model under minority owner Sir Jim Ratcliffe's INEOS regime.
"From what I have been told, United aren't looking at the free agent market," Manchester Evening News correspondent Tyrone Marshall said in a fan Q&A.
"Many of these players available are past their best; Ramos and Carvajal certainly fit that description. Others have been released because their games have regressed, such as Alaba.
"Essentially, they are free agents for a reason, but they are also players who in the past have been paid very handsomely. They don't fit the recruitment model that INEOS have introduced at Old Trafford."
Manchester United focused their summer spending on a midfield rebuild instead, investing more than £150 million in Carlos Baleba, Andrey Santos and Youri Tielemans following Casemiro's departure, leaving little scope to address the left-back position.
That has left manager Michael Carrick reliant on Luke Shaw as his only recognised senior option there, despite the Englishman's history of fitness problems and his contract at Old Trafford now entering its final year without a renewal agreed.
Marshall suggested United's stance could shift only in the event of an injury crisis.
"The only possible way this changes is if Luke Shaw picks up a significant injury and United are left with no left-back at all, but even then it's far from guaranteed," he added.
Alaba struggled with fitness during his final seasons in the Spanish capital, making just 32 La Liga appearances across his last three years at the Bernabéu, but did feature for Austria at this summer's World Cup, captaining his country to the round of 32 with four appearances in North America. The Vienna-born defender won the Champions League with two different clubs during a decorated career that also included spells at Bayern Munich, where he collected a host of Bundesliga titles before joining Real Madrid in 2021.
United are not the only club to have cooled on the idea of a reunion with Alaba this year. Reports had previously installed United as bookmakers' favourites to sign him following his Real Madrid exit, but interest has since gone quiet across the market, with his recent injury record thought to be a significant factor behind the lack of concrete offers.
United instead used a chunk of their summer budget on midfield reinforcements after Casemiro's exit and Manuel Ugarte's long-term injury, leaving little room to also address the left-back position before the window closed.