[slics]

Usually requirements are at a high level and if the previous team broke them down further in (scrum terms) user stories it would be more beneficial. If the stories are not available, then I would consider looking at the complete/done requirements first then worry about the new requirements. The intent is for you to understand what the application does and what functions are executed while you perform a given business process / workflow.

(Edit) Forgot to mention one important factor. Make sure to build a good relationship with your manager so that he understands that you are doing everything you can to make this work. The conversation is better understood when the relationship is there. Keep him aware of the work you are doing so he understands it as well, and he will have the data upfront to share with the customers.