By Sam McPherson

The San Francisco 49ers had a tumultuous offseason in terms of personnel changes, especially on defense, and as the team heads into the regular-season opening game on Monday night at Levi’s Stadium against the Minnesota Vikings, the only real injury issue for the team, unfortunately, comes on defense—and it’s kind of a big one. Top cornerback Tramaine Brock is listed as questionable for the game with a hamstring problem, but otherwise, the 49ers are very healthy for their Week 1 contest.

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Brock was a limited participant in both Thursday and Friday practices, and he’s listed as the No. 1 CB on the roster. If he can’t go on Monday night, the 49ers will be forced to bump up everyone else on the cornerback depth chart: Kenneth Acker would move from the nickelback to a starter, and Shareece Wright would move up to the nickel role. Dontae Johnson would then move up to the No. 4 corner position for the game. The secondary isn’t a strength of the San Francisco defense, and with the weakened front seven, it already was going to be under pressure against Minnesota quarterback Teddy Bridgewater on Monday night.

The front seven of the 49ers defense lost two linebackers (Chris Borland, Patrick Willis) and a defensive end (Justin Smith) to retirement, and then in August, the defensive line lost another player (Aldon Smith) for disciplinary reasons. Those four players have played big roles in the last four seasons of San Francisco’s defensive success, and the team is rebuilding the former strength of the defense. During the Jim Harbaugh era, the secondary always was the weak spot on the defense, and now in the Jim Tomsula era, it’s even weaker.

Brock played in only three games last season, but in 2013, he intercepted five passes. Keith Reaser is the other starting CB, but he’s never played in a regular-season game after spending his rookie season on injured reserve last year. The same goes for Acker: no real NFL game experience. Wright was a former starter in San Diego, and he has one career INT in 44 career regular-season games. He did get credit for 19 passes defended over the last two seasons with the Chargers, though. Johnson played in all 16 games last year for the 49ers, returning his one INT for a touchdown.

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The two starting safeties—FS Eric Reid and SS Antoine Bethea—are solid, but as a whole, the cornerbacks will be tested by Bridgewater and his wideouts (Charles Johnson and Mike Wallace). The Minnesota offense also gets All-Pro running back Adrian Peterson back from his 2014 legal issues, adding another crazy-hard-to-defend dynamic to the Vikings attack. Bridgewater drastically improved in the second half of his 2014 rookie season, and with more weapons at his disposal in 2015, he will be tough to defend.

Brock’s potential absence is truly the only injury concern the 49ers have this week, but it’s a big concern nonetheless due to the decimation of the S.F. roster on defense and the potential of the Minnesota offense.

Tramaine Brock, Cornerback (hamstring)

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Sam McPherson is a freelance writer covering baseball, football, basketball, golf, hockey and fantasy sports for CBS, AXS and Examiner. He also is an Ironman triathlete and certified triathlon coach.