Felgenhauer notes that it would be possible to keep conscripts for only a few months longer than their contracts called for — but that would run the risk of a drastic slip in morale as soldiers overstayed their expected conscription period.
Meanwhile, the arriving batch of Russian conscripts will have had limited training, reducing their battle effectiveness.
The incoming round of conscripts will serve within Russian airborne units, marines, and army brigades.
I don't understand why it should be will have had instead of simple will have. Could you please explain this grammar point to me?