In the Communist Manifesto, Marx defends the Hegelian dialectic. Or rather, he defends it against the attempt to break with it that is concentrated in capitalism. He exposes this attempt as really only a dialectical redoubling that does not escape from the dialectic.
We can see this in the passage where he recasts the Hegelian master/slave relationship into the bourgeoisie/proletariat relationship. The second is actually the truth of the former--that is, in less Hegelian terms, the bourgeoisie/proletariat relationship is the dialectical overcoming of the antagonism or contradiction between the master/slave relationship. Instead of exploiting the slave, the master here tries to take care of the worker so that the worker can continue to work. This allows both master and slave to work for the master's master, work itself. But what is crucial about this is that the "taking care of" here or "feeding" of the slave is only feeding the slave such that the worker's work--and not the worker himself--can continue. The emphasis is upon work abstracted from the existence of the slave that provides the work. Thus the slave sinks below the conditions that he would be under if he were wrapped up in the feudal master/slave dialectic, because the master here is not concerned with his existence--the master is "incompetent to assure the continued existence" of the slave, as Marx puts it. The slave cannot properly be a slave under capitalism. That is, it cannot be assured as to whether he will exist as a slave: his bare existence is threatened in the face of the abstract labor-power he temporarily embodies.
Thus what is going on is the attempt to dissolve the dialectic into a relationship where there will be no conflict, no dialectical conflict, between the master and the slave. A critique of capitalism, then would be the reassertion of the conflict or antagonism between the worker/slave and the master, and furthermore the assertion that the bourgeoisie is not a proper master. In other words, it would be a defense of the dialectic--not in order to resolve it again in the abstractions of Hegel, but to then direct it from this false escape from the dialectic into a socialist, and then communist, actual escape. I'll elaborate this more later. Here is the passage from the Manifesto--I've italicized the portion of the text that is explicitly under consideration: