The German word, that means what Kant actually wrote, is voraussetzen, e.g. in the preface of the Critique of Practical Reason, A13/14, Ak. 5:8 where he writes:
It presupposes indeed the Groundwork to the Metaphysic of Morals (GMM), but only to
the extent that this provides a preliminary familiarity with the principle of duty, and renders and justifies a determined formula of that duty. Otherwise this critique is complete
Es setzt zwar die Grundlegung zur Metaphysik der Sitten voraus, aber nur in so fern, als diese mit dem Prinzip der Sittlichkeit vorläfige Bekanntschaft macht und eine bestimmte Formel derselben angibt und rechtfertigt; sonst besteht es durch sich selbst.
voraus could be translated to beforehand or in advance.
setzen could be translated to set or put sth., but in this context it is better to say to assume.
As it has something to do with inference @virmaior is correct in saying that imply or necessitate would change directions, as it is rather that only because we assume something beforehand, we are logically enabled to make a certain statement or perform a certain act.
E.g. only because we presuppose the ball will fall, we throw it in an angle, not trying to throw it in a straight line to our aim. Our angle implies that we made this presupposition and the presupposition is necessary for achieving our apparent goal.
If you look at Guyer's words, they literally say:
In fact, Kant does not suggest that one has to assume the existence of something with unconditional value before being able to state the possibility of conditional value; rather, he assumes that morality requires the existence of something of unconditional value, and infers from this that conditional or relative value cannot be the whole story about value.
The comment of @MauroAllegranza provides good further readings on that topic.
Disclaimer: This answer does not say anything different from the other ones, just the same content in another form ;)