When things are going wrong, don’t make it even worse by justifications! Believe me it doesn’t matter! What matters to management, and should matter to you as well, are results!
Naturally, Results require actions. So your next step should be thinking of corrective actions before reaching your manager. Approach your manager with something like: “I believe we will not be able to finish on time. However, if we did so and so, we will reduce the impact”.
Justifications are not actions! Don’t approach your manager with something like “we will not be able to finish on time; anyway it’s all because of so and so!” it is completely out of context! Believe me manager do not appreciate telling bad news even if they are justified.
But, how to clear your yard? How to make sure that your manager is clear about whose mistake was that?
After the issue is resolved and things are back on track, call for a debriefing session and/or post-mortem analysis! Now you’ve the luxury to investigate and discuss what went wrong, how and why.
One last thing to remember is, to make sure not to personalize it! The focus should be on what you, as a team, have learnt from this experience and how to avoid similar issues in the future. It should not be about founding who’s accountable to punish him/her.