There were three Gretzky back-related injuries in 1990 to 1991. (I previously detailed them in another thread and showed the video of each one, but I can't find it right now.)
March 22, 1990 -- vs. the Islanders in L.A., Gretzky was sandwiched between two players, wrenching his back. He had to leave the game and didn't play again until the playoffs (missing 7.5 games).
April 22, 1990 -- vs. the Oilers, game three, in L.A., Gretzky was crunched from behind by Steve Smith and re-wrenched his back. He left the game, and that was the end of his season.
Sept. 14, 1991 -- vs. Team USA, Gretzky was nailed from behind by Gary Suter, wrenching his back. He left the game and didn't return for game two.
The September 1991 injury clearly had a massive impact on Gretzky's physicality, as his even-strength production took a massive nosedive from exactly this moment onward (from around 110 ES points a season to 60 overnight). His prime decisively ended with the Suter hit.