Professor Lost Assignment Grade, But So Did I
What you should do, in the unlikely event he does that is just go see him and work it out. There are easy ways to compensate, but your next step should not be an escalation or claim unfairness. Just go to the office and have a talk. 

The easiest way to compensate is just to assume that those grades are about the same as the typical grades you get otherwise in the course and compute averages or totals or whatever on that basis. If that satisfies both of you, then there is no issue. 

But if either expects something different, you just need to discuss it. Perhaps he asked a few questions about what you did on the assignment. 

Or, in case that you have the recorded grades, but not the assignments, just take the printed spreadsheet to show how you record things. He will accept it or not. And if you remember anything from feedback written on the assignment, say what it was, to refresh his memory. 

But if all the grades, not just yours, are lost, then he needs a general solution that isn't punitive.