Telemadrid (h/t Richard Fay of the Manchester Evening News) said the Real Madrid hierarchy is running out of patience with the Welshman and questioning if he delivers value for money for his high wages.
The situation may leave the door open for the Red Devils to pursue the superstar, with the Old Trafford club linked to Bale in the past.
Bale has suffered consistently with injuries in recent years, forcing him out of the starting lineup for extensive periods.
Mark Ogden of ESPN FC reported in June that Bale desired an extended stay in the Spanish capital after new manager Julen Lopetegui took the reins from ex-boss ZinedineZidane.
Ogden wrote United would be at the "front of any queue" if Real decided to put the winger up for sale.
United and Real have both lacked consistency this term, and the two superclubs will be searching for fresh additions to bolster their ranks.
Los Blancos have won only four of eight games in La Liga this season, and the Red Devils already appear out of the title race in the Premier League.
United's issues are deeper than adding a world-class attacker, but the purchase of Bale would lift the current gloom at Old Trafford.
The player is once again dealing with a groin problem, and his injury record will concern United coach Jose Mourinho.
However, a fit Bale would take English football by storm and deliver the Theatre of Dreams the talisman the fans crave.
Wales manager Ryan Giggs backed United's reported interest in the summer, but he questioned why Bale would leave the La Liga giants.
Speaking to the South China Morning Post (h/t Goal's Chris Burton), Giggs said:
"I'd be a happy United fan, that's for sure. I think he's a Man United player. I think the fans would love him. The fans would adore him but again, like I say, he's at a club where he's won three Champions Leagues, so why would you leave?"
The 29-year-old has three goals and two assists in seven La Liga appearances this season, but as he walks in the footsteps of Cristiano Ronaldo, more is expected from a player of his status.
Bale was brought to Spain to be Ronaldo's long-term successor, but the winger's goal ratio is not comparable to the Portuguese icon's.
United are on the verge of crisis, and the squad appears to be lacking confidence after a poor pre-season.
Mourinho is losing his grip in Manchester, and perhaps only a fresh wave of transfers will see the club reposition itself at home and abroad.
Bale needs a huge stage if he leaves Real, and United are one of the few clubs in the world that can offer him that kind of global attention.