The Bayern Munich manager's contract expires at the end of the current season and he is not expected to agree a new deal with the Bundesliga champions.
Guardiola has been linked with a number of top Premier League clubs including Chelsea and Manchester City.
Van Gaal's contract expires at the end of next season when the 64-year-old is set to retire. United will not cut short Van Gaal's deal just to make sure they get the ex-Barca boss, meaning Guardiola could take a year out of the game, just like he did in 2012 after leaving the Nou Camp.
However, rivals City could pounce and move for Guardiola next summer despite manager Manuel Pellegrini signing a new deal in August.