You still can depending on how you do it

Talk to the guy who has the dependency task, ask him/her to provide an interface that you can mock and he/she can implement against, you can have a Mock marker interface, your mock will implement the Mock marker interface and the dependency interface. The user of the interface can use reflection to detect the impelmentation classes of the dependency interface, and if there is an implementation that doesn't implement the Mock marker interface, use it. Otherwise, use the Mock interface implementation.

With this approach, you can seamlessly integrate the dependency impelentation without(or very little configuration change) any delay, and you can claim you finish the task you have and show on the burn down chart