"There's only one person who can survive a journey outside the TARDIS, Captain, and we both know it's not him." The Doctor turned around to face the Master fully, studying for a while as he considered. "I'll find you a room somewhere." Room, because it sounded better than cell and because, quite honestly, he very likely wouldn't throw him into a cell. The TARDIS however might. Gladly would, probably.

"I just can't let you leave it. Not yet." He had hopes, high hopes. In time, he'd be able to trust the Master. They were so much to each other. The Doctor also knew that Jack would disagree with his leniency, but then, if it wasn't for Jack he'd probably still let himself get beaten up.