Van Nistelrooy resigned in 2023 after winning the Dutch Cup and finishing second in the Eredivisie in his only season in charge - and he was not short of offers.
But, instead, he took a year out to learn from other coaches around the world at clubs such as Spanish giants Real Madrid and Argentine powerhouses Boca Juniors and River Plate.
"He's not a proud man in that way, he's not arrogant. He wanted to bounce his own ideas off others," Marcel van der Kraan, sports editor of Dutch newspaper De Telegraaf, told BBC Sport.