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The defending NFC Champions made their preseason debut against the Denver Broncos on Friday with mixed results. The San Francisco 49ers starters were impressive on both sides of the ball, but their backups along the offensive line and at quarterback struggled a great deal.

For a team that has been hit with the injury bug, San Francisco simply cannot afford anymore injuries at key positions. While being one of the deepest teams in the NFL, certain positions are definitely lacking here.

With that in mind, rising star Colin Kaepernick looked about as sharp as possible in his initial drive of the 2013 preseason. He connected on all four passes that he attempted and scrambled to avoid pressure on a key third down; gaining a half of dozen yards and first down.

Kaepernick connected with new wide receiver Anquan Boldin two different times on the initial drive and hit Vernon Davis on an out pattern later on. He was keyed in for that one possession, looking extremely comfortable in the pocket.

This comes on the heels of reports from Santa Clara that have indicated that Kaepernick has improved a great deal from what was an astonishing sophomore campaign in 2012. If Friday was any indication, other NFL teams have to be on watch.

Speaking of Boldin, it seems that he found an instant rapport with the young quarterback. Despite being blanketed on one of his receptions, the veteran gave Kaepernick a lane to throw through and made a nice catch. This was an all-too-familiar sight for 49ers fans in the Super Bowl when Boldin was suiting up for Baltimore.

San Francisco’s first-team offensive line dominated the Broncos starters on defense and opened up some nice rushing lanes for second-year running back LaMichael James, who started in lieu of the resting Frank Gore. James tallied 27 yards on eight attempts. Fellow running back Anthony Dixon added 27 on seven attempts.

Defensively, San Francisco didn’t allow a single point until three minutes remained in the final stanza. Denver’s sole touchdown came on a D.J. Harper fumble that Shaun Phillips took to the house in the second quarter.

Let’s take a look at some standout players, either good or bad, from San Francisco in its preseason opener.

The Good

Colin Kaepernick, Quarterback: The third-year quarterback looked poised in the pocket and actually showed some great finesse in limited action Friday. San Francisco couldn’t have expected much more from Kaepernick in this one. It would have been nice to end the opening drive with a touchdown, but a false start penalty by Anthony Davis blew that opportunity. Overall, a solid performance from No. 7 in this one.

Anquan Boldin, Wide Receiver: As expected, the new addition made an impact in his initial game with the 49ers. Boldin hauled in two passes for 14 yards on two targets. He looked very much like a No. 1 receiver and will have to keep it up in order to come close to replacing Michael Crabtree’s production from a season ago. In any event, it was telling just how good Kaepernick and Boldin looked together against Denver’s first-team defense.

Vance McDonald, Tight End: Your leading receiver from the opening game; there was a whole heck of a lot to like from the rookie second-round pick in this one. The Rice product put up 66 yards on four receptions and showed why his athleticism will make him a tremendous asset for San Francisco in 2013. He created mismatches all evening long; whether it was going up against safeties or linebackers. You can expect the 49ers to continue utilizing his rare set of skills all over the field as the preseason goes on. I loved what McDonald showed in this one.

Corey Lemonier, Linebacker: “Pure beast mode.” That’s probably the best possible term to explain Lemonier’s performance in his initial NFL game. The rookie from Auburn may not have filled up the stat sheet (zero tackles), but he got tremendous pressure throughout the game. His ability as both a stand-up pass rusher and with his hands down at the line will help San Francisco’s depth in the front seven in 2013. If Friday’s performance was any indication, Lemonier will be a huge surprise as a rookie.

Parys Haralson, Linebacker: Speaking of surprises, Haralson’s performance came out of absolutely nowhere. After missing the entire 2012 season due to injury, the former starter found himself in the game with backups. He recorded five tackles, one sack, two tackles for loss and a single quarterback hit. Haralson looked stronger and more athletic than we have seen in the past, and should be able to make an impact in 2013. After all, he did record a total of 19 sacks in a four-year span from 2008-2011. San Francisco could sure use that experience behind Aldon Smith and Ahmad Brooks at outside linebacker.

Nnamdi Asomugha, Cornerback: It amazes me what being on the roster bubble can do for a veteran. At times as a Philadelphia Eagle over the past two seasons, Asomugha looked disinterested in the defensive backfield. It just didn’t seem that his heart was into it. This coupled with a lack of a match as it relates to scheme fit, caused tremendous struggles for the former Pro Bowler in Philadelphia. This wasn’t the case Friday evening. Asomugha played quite well with the 49ers’ second-team defense, showing some ridiculous physicality and solid back-peddling ability. If Asomugha can even come close to regaining old form, he’ll upgrade what has to be a questionable secondary in San Francisco.

Eric Reid, Safety: The rookie first-round pick may not have started, but he did get a vast majority of the playing time at free safety. Overall, Reid showed great range in coverage over the top and did a solid job reading routes down the field. In addition, Reid laid the wood a couple different times, which reminded me a great deal of former 49ers’ safety Dashon Goldson. Reid recorded six tackles in extended playing time. Though Craig Dahl got the start in this one, Reid will likely be sitting back there Week 1 against Green Bay.

The Bad

Second-Team Offensive Line: Ugh, this was pretty hard to watch. Whether it was Patrick Omameh or Joe Looney, San Francisco’s backups along the offensive line were beyond putrid. The issues included missed blocking assignments, complete whiffs on other blocks and getting thrown back behind the line of scrimmage. Harper’s fumble, which led to Denver’s only touchdown of the game, was in part caused by pretty bad blocking upfront. If San Francisco does suffer an injury along the offensive line, it could be in trouble during the regular season. As it is, the 49ers are stacked as it relates to the first unit here.

Back-Up Quarterbacks: Colt McCoy injured his shoulder, but was able to return. It really didn’t matter, as he struggled finding receivers on a consistent basis throughout the day. The former Texas Longhorn completed just three-of-seven passes for 41 yards and an interception. Blocking was an issue here too. Despite only being sacked once, McCoy faced pressure throughout his outing. In reality, he didn’t play too well against said pressure. Scott Tolzien, on the other hand, did do some good things. He completed 15-of-26 passes, but also threw an ill-advised interception and wasn’t able to lead San Francisco down the field on a consistent basis.

Vincent Frank is an NFL featured columnist at Bleacher Report.

Vincent is the head sports editor at eDraft, co-host of eDraft Sports Radio (which airs every Monday and Wednesday from 3-6 p.m. ET) and a fantasy writer for Pro Football Focus. He’s also the news director here at PFC and co-host of Football Debate Central with Ryan Riddle8:00-9:30 every Wednesday.