We lost for a lot of reasons, Rondo's injury being one of them.
Other reasons (not in any particular order):
* Lack of post offense, or any timely offense
* Lack of post / penetration defense
* Lack of rebounding, especially toward the end of the game
* Injuries to Shaq, Delonte, JO, etc.
* Doc's rotations
* Failure to execute down the stretch
* Fatigue / age
* Amazing shots by Lebron and Wade
* Failure to defend James Jones
* KG's 1-for-12 in Game 4
* Lack of home court advantage
* The Perk trade
* Lack of contribution overall from the bench
* BBD's utter failure to show up
* The refs
I'm not willing to say that we would have won with a healthy Rondo, since we fell down 0-2 with him in the lineup. He would have helped, but we would have probably also needed to have multiple things listed above also go our way.