Rule trigger only when a specific field is (or is not) changed?

I have a node type which can have a user assigned to it (via an entity reference field within the node), and I'm trying to create a rule which will notify the user when a node assigned to them is updated. The obvious solution would be to simply set the trigger to "when existing content is updated", but the issue is that the nodes will often be created without a user being assigned initially. This would mean that when the node is later assigned to a user, it will tell them that a node assigned to them has been updated rather than notifying them that it has been assigned to them in the first place. So my question is, is there a way to have a rule that will trigger only if the assigned user field was not changed when the content was updated? This is with D7.