Sure, but it's not everything.
Take, as a perfect example, the 04-05 Panthers and 04-05 Eagles.
The 2004 Panthers were riddled with injuries. Steve Smith, Kris Jenkins, you name 'em. They were able to battle back, but still finished out of the playoffs at 7-9 a year after reaching the Super Bowl.
In the offseason, they lost Willig, Tillman, and Muhammed. They gained Bashir (injured late), Lucas, McCree, and Wahle. But in 2005, with their only big injury coming to Jenkins (again), they went 11-5 and made it to the NFC Championship.
The 2004 Eagles had only a handful of injuries (Ritchie, Andrews, Buckhalter, Kalu), went 13-3, and made it to the Super Bowl.
In the offseason, they lost Reese, Simon, Burgess, and Mayberry, while re-signing Westbrook and Trotter and picking up Gordon, McCants, Thomas, and Strickland. However, they were absolutely riddled with injuries to McNabb, Westbrook, Thomas, Sheppard, Buckhalter, Pinkston, Fraley, Reed, and McDougle. They finished 6-10 and in 4th place.