HELLO THERE!
Its straight in this senario , i think these are the important reasons for power loss:-

1. DVT is a life threatening disease with sharp pain in calf, patient has strong tendency to become non ambulatory, leading to DISUSE ATROPHY.

2. DVT compensates proper venous circulation in gastro-soleus leading to accumulation of waste metabolites , which keep on accumulating and leading to improper nutrition of gastro-soleus hence atrophy and weakness.

ONE WORD OF CAUTION:- IF THAT PATIENT HAS STILL BY ANY MEANS MINOREST SIGN OF DVT ON DOPPLER STUDY, PHYSIOTHERAPY IS SHARPLY CONTRAINDICATED.
DO NOT ATTEMPT TO TREAT HIM.