On Monday the military judge dismissed one of the most serious charges of bodily injury against Drake, reducing the maximum possible punishment from 21 1/2 years to 16 1/2 years.
Drake may also get demoted, be dishonourably discharged and have to forfeit all pay and allowances.
Drake, a food-service sergeant at the time, was accused of striking the detainee in December 2003 at a US military base near the Iraqi town of Falluja.
Specialist Angel M Bonilla testified that he saw Drake strike a detainee with an aluminium bat, saying Drake told him afterward, "B, come get a piece of this".
Bonilla said he grabbed a bat and struck a second detainee, regretting his act but saying he felt pressured to do it.
Only one defence witness took the stand.
Major Michael A Cortines, an Army physician, testified that detainees at the base received careful medical inspections when they arrived at the temporary holding facility and were examined daily by medical personnel.
Michael Waddington, the defence lawyer, said Cortines's testimony supported the claim that no medical evidence existed that anyone was abused at the base.