The 31-year-old recently admitted that he will make a decision this summer about where he will play his football for the upcoming campaign.
A move to his boyhood club Everton, as well as switches to MLS and the Chinese Super League have been mooted due to his game time at Old Trafford dwindling.
The forward started just 15 Premier League matches last season under manager Jose Mourinho, and his lack of minutes resulted in him being left out of the last two England squads.
It appears that Rooney is keen to get in the best shape possible for the new season as PA reports that he has already started training, despite United's pre-season schedule not beginning until July 9.
Rooney has been at the North-West club for the past 13 years, winning 12 major trophies, including five league titles.