Quick answer, get NM elsewhere. A little costal fort busting can get you some easy NM.
That said, you are still better off loosing 10 NM than if you badly loose 1st bull run trying to take Manassas. As long as you build enough troops, the NM hit is not going to kill you.
The trick when behind in NM is caution. Build up, defend, and attack when you have a local superority. The nice thing about the Union, is that even if you are down 20 NM, you should bé able to outnumber the rebs enough that you should be able to defend. If the CSA is down 20 NM... things get hard.