I don't think it's gamemanship. At least, it has been confirmed by other Celtics reporters, and apparently KG's ankle is hurt, not his knee. 
ESPNForsberg : 
  
KG (ankle) and Perk (knee) both sat our today's practice. KG likely not practicing tomorrow, Perk could be back tomorrow. 
Celtics : 
KG (ankle) and perk (hyperextended knee) both missed today's practice. KG was hurt at end of Game 2, Perk injured at shootaround.