One potential roadblock to injecting a woman's cells into a man's testicles is that the man's immune system might attack and destroy the woman's cells. In usual circumstances, when foreign cells (such as cells or organs from other people, or infectious bacteria) are put into a human body, its immune system will reject such cells or organs.
A significant hurdle in transplanting a woman's cells into a man's testicles is that his immune response may target and eliminate the foreign tissue.