Mandzukic was linked with a move away last summer.
“Mario is an incredible guy. I adore him," Agnelli told La Gazzetta dello Sport.
“There's not much else you need to say about him. He wants to win every competition he takes part in, just like us here at Juventus.
“He discussed the situation and at times players do need time to reflect. We must all applaud what Croatia achieved, as reaching the Final is remarkable considering the size of the population.
“Croatia have champions in many sports and are recognised on a global level for that. Mario is one of their greatest talents and has immense determination."