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We consider the following primitive relay channel. A source X wants to send information to a destination Y with the help of a relay Z. The link from Z to Y is assumed to be lossless with rate R ₀ . It is shown that destination Y's capability in simulating relay Z's observations, a function of the channel, is fundamentally linked to Z's capability in utilizing the full rate of the ZY link. A general upper bound based on combining channel simulation and the blowing-up lemma is presented. In several cases, e.g. stochastistically degraded, we show that the rate C _XY + R ₀ is not achievable, where C _XY is the capacity of the XY link. This method reveals an interesting relationship between channel simulation theory and characterizing the relay channel's capacity.