Grant Cohn: 49ers' run defense has problems

You can dismiss how easily the Ravens' offensive line pushed around the 49ers' front seven Thursday night. You can rationalize the Niners giving up 237 rushing yards by pointing out they had only six healthy defensive linemen, and Justin Smith and Aldon Smith and Ray McDonald and Patrick Willis didn't play. You can simply forget the whole thing because it was a preseason game and those don't count.

Let's try to look at the game objectively. What do we really see?

The coach didn't look happy on the sideline. He looked like he had eaten bad food.

And his front seven looked like they had Velcro on their jerseys. They got stuck on the Ravens' blocks all game. It's not a sin to get blocked, but it is a sin to stay blocked, and the 49ers' front seven stayed blocked Thursday night. They got blocked, manhandled and pushed around by one of the worst offensive lines in football. The Ravens ranked dead last in yards per rush last season.

Maybe that's why Jim Harbaugh looked sick. It's disturbing to give up 200 rushing yards in any game to any opponent, especially the worst rushing offense in the league. This was the first time the 49ers had given up 200 rushing yards in a game since Harbaugh took over. The last time the 49ers gave up 200 rushing yards was in 2010 under head coach Mike Singletary. The Chiefs rushed for 207 yards against the Niners Week 3. That was the season before NaVorro Bowman became a starter.

Now we arrive at the key issue – NaVorro Bowman.

Since Bowman became a starter in 2011, he has been the 49ers' best linebacker – better than Patrick Willis. Bowman has been the 49ers' best defensive player regardless of position. Bowman has been the 49ers' best player, period.

Bowman is not coming back soon. He tore his left ACL and MCL during the NFC championship game in January and will miss most of the season, maybe all of it.

His replacements played terribly Thursday night and don't delude yourself into thinking it was a mere exhibition game and the results were not revealing. They were. Michael Wilhoite, Chris Borland and Nick Moody did not did not fight off blocks. The Ravens steamrollered them. Ray Rice averaged 5.7 yards per carry, and Lorenzo Taliaferro, a rookie fourth-round pick from Coastal Carolina, averaged 5.5 yards per carry. The 49ers made him look like Jim Brown.

The 49ers have had the best front-seven in football since Bowman became a starter in 2011. Will the front seven still be great without him? If Bowman was the only guy the 49ers had to replace, maybe. But there are others.

According to Matt Maiocco of CSN Bay Area, the 49ers expect the NFL to suspend Aldon Smith for the first six to eight games of the season.

Justin Smith had shoulder surgery a few months ago and has not practiced all offseason.

Ray McDonald had biceps surgery in January and has not participated in training camp.

Glenn Dorsey tore his biceps last week and will miss most, maybe all of the season.

Ian Williams broke his ankle last season and is still rehabbing it.

Best case scenario, the 49ers open the regular season starting three defensive linemen who have had no offseason and are on the mend – Justin Smith, Ray McDonald and Ian Williams.

Just two starters in the 49ers' front seven are healthy and have been healthy all offseason – Willis and Ahmad Brooks. That's two of seven.

The past three seasons, Willis and Brooks played on the same side of the defense. They played on the left side, the strong side, the side with the tight end. Bowman, Aldon Smith and Justin Smith played on the right side.

With Bowman and Aldon Smith out, the 49ers have a major weakness on the right side of their front seven. Justin Smith is all alone over there. He turns 35 in September – he needs help. So the Niners are moving Willis over to that side, the side Bowman used to play. Is that enough?

Wilhoite, Borland or Moody has to start next to Willis at inside linebacker. None of those three currently is a good option. Mike Purcell has to start at nose tackle until Ian Williams can play – Purcell was terrible against the Ravens. And Corey Lemonier has to replace Aldon Smith. Lemonier wasn't terrible, but he did nothing special Thursday night. He's just another guy.

For the first time in the Harbaugh Era, the 49ers' front seven isn't the strength of the team. The front seven used to shut down the opponent's run game and force it into tough passing downs like second-and-8 or third-and-8. Then, the 49ers' front four created tremendous pressure on the quarterback – they didn't need blitzes. All of this covered up obvious flaws in the 49ers' secondary.

If the front seven is merely ordinary this season, the whole defense could be in big trouble.

You can't dismiss what happened in Baltimore as a mere exhibition game fluke.

Grant Cohn writes sports columns and the 'Inside the 49ers' blog for The Press Democrat's website. You can reach him at grantcohn@gmail.com.