For the last three weeks of the season, San Francisco 49ers defensive lineman Justin Smith had to sit out with a triceps injury. He was so dominant this season that he was voted All-Pro at both defensive end and defensive tackle. Whether he's playing on the outside or the inside in the Niners' base 3-4 scheme, he's an absolute monster.
His 66 tackles and three sacks don't even come close to illustrating what Smith means to his team. He's a rare two-position, every down freak who's equally effective in any defensive line position, and against both the pass and the run.
If you haven't heard the Aldon Smith stat by now -- if you missed getting hit over the head with it, congrats on avoiding almost all media this week -- the Niners' pass-rushing outside linebacker had 19.5 sacks in the first 13 games of the season and looked like he had a very good chance at breaking the single-season sack record. Then, Justin Smith got injured, and Aldon didn't register a single sack during the final three games of the season.
The Niners' DBs are good, but they're not so good that they can shut down the Packers' wide receivers without a very significant pass rush. If Justin Smith can't play his normal allotment of snaps or isn't as effective as usual, Aldon Smith isn't going to be terribly effective. If the Niners give Aaron Rodgers more than a couple of seconds to throw on any given play, he's probably going to find an open receiver.
Similar things can be said about Patrick Willis. While he's an All-Pro on his own merits and would almost certainly be an effective player on a very poor team, he's allowed to truly dominate games because of the play of the defensive line in front of him. If Smith isn't occupying blockers and/or getting into gaps, forcing the opposing running back to improvise, Willis isn't going to be nearly as effective as he normally is.
Who wins or loses Saturday night's game might come down to the health of Justin Smith. If he's anywhere near his best, the Packers' offense could struggle to move the ball. If he's limited in any way, it will affect the 49ers' DBs, Aldon Smith and Patrick Willis. He's listed as probable and will play with a brace on his arm, but there's no way of knowing how healthy he is or what he's capable of until he gets out onto the field, playing at playoff intensity.