I can't help but think that Brad Stevens has zero problem with 4 of his top 5 players spending extended off-season time playing and learning under Gregg Popovich.
It would be the equivalent of Brown's coach, Freddie Kitchens, sending Baker Mayfield, OBJ, Myles Garrett and Nick Chubb to spend a month and a half of the off-season training, playing and learning under Bill Belichick.