He nodded, took that information in, filed it away for later. "That's fascinating, and normally I'd want to ask quite a lot of questions about that, have a nice a little chat, really I would," he babbled. "But I need to find Koschei quickly. I just. I do. I need to fix this."
He need to be the one who found the Master, not the other way around. And the Doctor was was a little terrified of Jack finding the Master before he did. Jack was already on the Master side in this, and Jack probably would try not to get in the middle, but the Doctor knew he'd done something quite hurtful and. Jack's best intentions might go out the window.