Several media outlets have reported that Kevin Garnett and Kedrick Perkins did not practice today. While Perkins seemed to have hyperextended his knee during practice, KG was hurt during Game 2.
During his weekly radio interview, Danny Ainge said that KG landed on Perk's foot at some point during the game. The video below shows that very play right before halftime and you can clearly see KG grimacing (it's the one good observation Reggie Miller actually points out the entire game).
There was a play at the end of the 4th quarter where KG seemed to maybe aggravate the injury. But this seems to be where it initially happened.