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Before we get to the 49ers’ upcoming preseason game, here’s a brief history lesson.

In 2010, the 49ers’ defense was ordinary. It ranked 13th in yards allowed and 16th in points given up. Four current starters were key contributors on that 2010 defense — Patrick Willis, Justin Smith, Ahmad Brooks and Ray McDonald.

After the 2010 season, the 49ers drafted Aldon Smith and signed Carlos Rogers and Donte Whitner. Tarell Brown and NaVorro Bowman, who hardly played for the Niners in 2010, became key starters in 2011. And the defense became one of the best. It ranked fourth in yards and second in points that season, and it ranked top-five in both categories in 2012 and 2013.

Rogers, Whitner and Brown are no longer Niners — they signed with different teams this offseason. Bowman will miss at least the first half of the season rehabbing a torn ACL and MCL. Aldon Smith could miss the first half of the season, too — the NFL hasn’t yet announced how many games it will suspend him.

Without Smith, Bowman, Rogers, Whitner and Brown will the 49ers defense revert to mediocrity? The answer depends on their replacements. A few of them will play extensively against the Broncos on Sunday.

Replacement No. 1: Corey Lemonier. He’s replacing Aldon Smith, arguably the best pass rusher in football. The 49ers did not have a strong four-man pass rush before they drafted Aldon Smith. He changed that right away. His first practice as a 49er, he made veteran left tackle Joe Staley look silly. Staley couldn’t get his hands on him. Smith has longer arms than most offensive linemen and he has quick, powerful hands like Bruce Lee. Smith practically karate chops offensive linemen’s arms. They can’t touch him, let alone block him.

Lemonier has none of that ability. He’s a good athlete, about the same size as Aldon Smith, but he doesn’t have Bruce Lee hands. Offensive tackles easily tie up Lemonier. In practice, he struggles to beat even the backup tackles like Carter Bykowski. Last Thursday, the Ravens occasionally blocked Lemonier with a rookie tight end named Crockett Gilmore and got away with it.

Lemonier played 70 snaps against the Ravens and recorded one sack on a play in which backup safety Bubba Ventrone blitzed. Lemonier was ineffective when the 49ers didn’t bliltz. The entire 49ers’ four-man rush was ineffective, actually. All three of the 49ers’ sacks came as a result of blitzing.

Watch Lemonier’s pass rushing technique against the Broncos. Can he beat first-string offensive tackles? Can he beat second-string offensive tackles? Can he beat blocking tight ends? If not, the 49ers’ pass rush will be in major trouble when the regular season starts and defenses game plan to exploit him — they may be able to block him with a tight end and therefore double-team Justin Smith.

Replacement No. 2: Jimmie Ward. He’s replacing Carlos Rogers as the nickel back, the corner who covers the slot receiver. Ward should be an immediate upgrade over Rogers as a run defender and a blitzer. Ward was a safety in college and hits hard for a shrimp. But can he cover? For what it’s worth, he has not covered well in training camp. He always seems to be a step behind the receiver downfield. He hasn’t been able to handle quick little Devon Wylie who probably won’t make the team.

Ward will match up on Sunday against Wes Welker. Welker is Ward’s toughest test of the offseason so far.

Replacement No. 3: Chris Culliver. He’s replacing Tarell Brown as the starting right cornerback. Culliver can shut down the best wide receivers on short and intermediate routes. During Friday afternoon’s practice in Levi’s Stadium, Culliver erased Michael Crabtree, didn’t give him an inch of separation.

But Culliver gets beaten deep by anybody who runs fast. On Friday, he got beaten deep by L’Damian Washington, an undrafted receiver whom the Cowboys cut a few weeks ago.

As the starting right corner, Culliver will match up frequently against fast wide receivers — offenses tend to put their deep-threat receivers on that side of the field. The first four weeks of the regular season, Culliver will cover Terrance Williams, Alshon Jeffery, Michael Floyd and Riley Cooper. All four averaged at least 16 yards per catch last season. The last time Culliver faced Floyd, Culliver gave up 5 catches, 89 yards and a touchdown.

Culliver will face Demaryius Thomas on Sunday. Thomas is one of the best and most complete receivers in football. He can get open short, intermediate and deep. Can Culliver hold his own against him? He’d better. He will face Thomas again Week 7.

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.