The Dutchman was in attendance at Selhurst Park on the final day of the Premier League season on Sunday, when Man United fell to a 1-0 defeat against Crystal Palace, finishing the campaign in sixth spot.
Next season's campaign begins on 6 August due to the World Cup being held at the end of the year, and Ten Hag is said to be calling the first wave of players back into training in five weeks time.
According to The Telegraph, those Man United players on international duty will have staggered returns to the club for pre-season, but the new manager is certain to increase the fitness levels of the squad.
Ralf Rangnick will leave his role in the dugout at Old Trafford with the lowest win percentage of any Man United manager in Premier League history, and claims that after being brought in suddenly he had no time to get the team to the physical level required to carry out his pressing style.
Ten Hag will now attempt to raise the fitness levels of his new squad, with their first pre-season game against Liverpool on 12 July.