You may recall that sources had the Celtics turning their focus after missing out on Caron Butler toward improving on House’s defense. In this regard, some close to the Knicks did a double-take when told that was one of the reasons the Celts wanted Robinson. But he is seen by some as a pretty good defender on the ball, and some feel he can maintain at least to some smaller degree the type of pressure Rondo presents. And with the way the Celtics use rotations and schemes, it is hoped he will fit into the formula.
A huge part of defense is athletic ability. Just ask Dennis Rodman. Nate Robinson isn't lacking in this department. For NBA players, another component is height. Smaller players get shot over, passed over, and dunked over. Nate is 5'9", tiny by pro basketball standards, even for a point guard. So even if Nate had the will and desire to play defense, he's starting out with one sizable disadvantage.
The question then becomes how much he wants to play defense and how much his teammates coerce him into playing the defensive sets. At the very least, small point guards can be a pain in the ass defensively, when they wanna be. Hopefully, Nate wants to be.