Transubstantiation is not, then, one disconnected and esoteric doctrine that a Catholic can take or leave. It is an articulation of faith in the real presence of Christ in the Eucharist seeking understanding in the theological context of a deep appreciation of the relationship between God and creation. To affirm it is to affirm something foundational to the whole Christian worldview. Not to put too fine a point on it: it affirms that God is God and creation is creation.
Transubstantiation is more than the doctrine of the Real Presence. It is a theological explanation of the latter.
Church Life: Transubstantiation Isn't a Disconnected Doctrine by Brett Salkeld