From: Iowan in MD/DC
However, if the unit is scheduled to be withdrawn at a later date, you may not be able to rebuild it. I do not know if this is the case with a sub unit. Usually, if you lose regiment out of a division you just recombine the other two and can then re divide it into three regiments that are about 2/3rd strength each.
If it's a regiment, I think you have to buy it back before you can combine... but if it's the A/B/C parts of a division, you should just be able to combine and split again.
I've had this happen with the Australian divisions, particular the one(s) with parts that start in New Guinea and the Gilberts and such. It's somewhat common for the pieces at Moresby, Nauru, etc., to lose those pieces to early invasions before you can pull them out. I'm 90% sure you have to purchase them out of the unit buyback list.