Nick doesn't seem to think The Doctor's off his rocker, at least when he's well aware that he's likely got a few holes in his own memory. But this guy isn't exactly made from the same stuff as Nick is, and the old detective is going to take this serious as he stands there with a serious look on his plastic face.

"Now that is a problem. Not just your missing friend, but the fact that you somehow had your memories tampered with. And I take you don't remember who did this to you." He sounds concerned, really concerned.

"Is there anything you might make you remember her?" He's not a doctor, but the detective in him doesn't see any harm in trying to jar The Doctor's memory loose.