Naturally he wants to help. Right, that's what decent people do. Sebastian is tired enough that for a moment he wonders if he's gone completely mad, seeing giant talking mice... before his brain catches up and he recalls seeing this person around before. Funny; this can't even make his top ten list of strange things he's encountered in his very long life. It's probably only number... something on the list.
"Oh, I am quite unhurt." Sometimes he still misses being able to lie. Case in point: right now. He can't honestly say he all right, after all. "Thank you for your concern, sir, but it's unnecessary."
To Sebastian's credit, having company means he snaps into focus once again. What kind of butler would he be, after all, if he went around looking tired among people*?