The Doctor regretted his words against Jack almost immediately after they left his mouth, but it was too late to retract them. He expected that Ianto wouldn't be too pleased with him. He knew that if anyone insulted Rose or the TARDIS and said they were wrong, he would be very upset.
What he hadn't anticipated was the younger man punching him in the jaw. It happened rather quickly, the Doctor couldn't even say for sure that he had seen Ianto's fist move. He just knew that one second, an angry look had crossed Ianto's face, and the next, pain was blossoming from his jaw.
The Doctor stumbled back from the force of the punch, just barely managing to catch his balance as he listened to Ianto's rant, his eyes getting wider. "What?" he said. The Master had done all those terrible things to his -- uh, to Jack? And Jack had let that happen to him, just to keep the Doctor safe?
The Doctor listened to Ianto, his brow furrowing. "I never realized that Jack had a brother, no." he said quietly. He licked his lips, rubbing his hand over his jaw. He also hadn't realized that Jack had cared so much for him. He'd just thought that Jack fancied him because, well, he was dashingly handsome and Jack flirted with anyone and everyone. He'd never once thought that it could be more, and now it was too late.
He cleared his throat and straightened up, working his jaw back and forth to make sure that nothing was broken. "Well, this is certainly awkward." he said. "Apparently today is the day for me to be continously mistaken about a great many things. Logistically speaking, he is not supposed to exist. He was supposed to have died at Satellite 5, but I suppose it is possible that everything happens for a reason."