Updated: Mar 14, 2021
Without Hercule there, Buu wouldn't really want to fight. But he remembers what Hercule told him in the days leading up to the Tournament of Power(TOP) and starts attacking. He has been training for this, so he would be strong enough to hold his own against whatever the other universes threw at him. He would turn his opponents into candy and fight the urge to eat them during the match.
Eventually, he would come face to face with his toughest opponent yet… Jiren. Buu would fight, but nothing he did would hurt Jiren. He finally gives in and tries turning Jiren into candy. But Jiren bounces the beam back and Buu is turned into candy. Jiren picks him up and saying, “how childish, don’t waste my time.” He then throws the candy out of the ring. Whis turns Buu back to normal in the bleachers, and Buu is disappointed in himself that he let Hercule down. Universe 11 ends up winning the TOP, but Jiren was so impressed with Goku when he fought him in his Mastered Ultra Instinct(MUI) form, that he gained a lot of respect for the Saiyan. He made his wish to the Super Dragon to bring back Universe 7 only, so that he may get another chance to face his newest rival Goku under more forgiving circumstances.