substitute, surrogate (person that acts as a substitute); substitute (player who is available to replace another if the need arises, and who may or may not actually do so)
to suppose, to surmise, to assume, to guess, to imagine, to figure, to say, to take it; to presume, to expect; to comprise, make up; to pose (e.g. a risk, a threat, a danger); to mean, to imply, to represent