"I didn't say I invited you to travel with me, not exactly." The Doctor shrugged. "I had said, at the time, after playing in your satellite system and ruining your grand plans for universal domination and destruction, that you were my responsibility. You would have been on the TARDIS, with me, and I would have eventually stopped locking you in the back of it."
After he regenerated, and he couldn't say if that would have been sooner or later than it had been with the Master dying in his arms. Well, if the Master hadn't convinced him to do so sooner. Letting him out, that was, not regenerating.