The Real Madrid player has been voted out of the fans' preferred eleven in Spain, and Real are struggling under their new manager Rafael Benitez. He has been booed and attacked by Real fans in the past, and might regard this as a good time to move on.
He would be guaranteed first-team football under Manchester United boss Louis van Gaal, where he would be capable of playing on either wing, or ahead of Wayne Rooney as a striker.
United might be willing to soften their stance on David de Gea should Real make a bid, using the interest as part of a move for Bale.
The report also suggests that United could use Phil Jones as a makeweight in a bid for John Stones, should United decided to make a move. Previously it had been expected that Jonny Evans might have been used, but he has also been linked with West Bromwich Albion.