The Manchester Evening News says Harry Redknapp's departure has cast doubt on Bale sticking around at White Hart Lane after another season out of the Champions League.
Barcelona and Real Madrid are both admirers of Bale, while Ferguson and Manchester City's Roberto Mancini would also be expected to enter the bidding if given the slightest indication that he wants out.
United are in the market for top class attacking options having tried and failed to land Eden Hazard.
Bale was not expected to figure prominently in United's transfer business this summer, but they could be prompted into action if he looks set to move on.
Of more immediate interest to Ferguson is the future of Modric – a player he has tracked for the past 12 months.
He baulked at Spurs’ £32m valuation of the playmaker – currently on international duty at Euro 2012 – but would be interested in doing a deal at a reduced price.