Final answer:
The patient's difficulty reaching the back of their head and lacking upward rotation is most likely due to weakness in the serratus anterior muscle.
Step-by-step explanation:
The patient is lacking upward rotation and is having difficulty reaching the back of their head. Based on the given information, the weakness is most likely due to a weakness in the serratus anterior muscle. The serratus anterior muscle plays a key role in upward rotation of the scapula and is responsible for allowing the arm to reach positions higher than the shoulder.