New Red Devils chief Louis van Gaal – who is currently coaching his Holland side for the World Cup – is set for a busy few months of transfer activity at Old Trafford, and has made securing a top class left-back a priority.
Luke Shaw of Southampton was seen as the major target, but negotiations for a £27m deal have been moving slowly in recent weeks.
It has been suggested that a deal will be done, but now United are eyeing alternatives.
Alba is thought to be seen as an ideal option, and a tentative approach has been made to Barca to test if an agreement will be able to be reached.
The Premier League giants are aware that convincing the 2012 European Championship winner to make the switch may be difficult, but they are hopeful given that the Catalans are set to undergo major surgery on their first-team in the coming months after a difficult campaign.
Alba has been a key man at Barcelona in recent seasons, yet his role under new boss Luis Enrique is unclear.