ABOVE: Cam Inman and Dieter Kurtenbach talk about the 49ers’ win versus the Titans

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The 49ers have their quarterback of the future. Jimmy Garoppolo has made that undeniably clear over the last three weeks.

But Sunday’s game against the Tennessee Titans made it painfully obvious what the 49ers are missing from their surprisingly robust list of foundational pieces: a No. 1 cornerback.

This 49ers’ roster is absolutely a work in progress, but it’s not the worst roster in the NFL anymore (congrats Indianapolis!) — head coach Kyle Shanahan and general manager John Lynch have some strong talent to work with in the years to come. Players like:

• Marquise Goodwin and Trent Taylor, who should be catching a ton of passes from Garoopolo in 2018 and beyond.

• Trent Brown and Joe Staley, who should be the bookends of the Niners’ offensive line, protecting Garoppolo, going forward.

• DeForest Buckner, who has proven himself to be an impact player and possible All-Pro on the defensive line

• Solomon Thomas and Arik Armstead, who should be reaping the benefits of the chaos Buckner creates up the middle.

• K’Waun Williams, who should be playing in the slot for the 49ers defense for the next half-decade.

• Safeties Jaquiski Tartt, Eric Reid (should he be re-signed), and Adrian Colbert, all of which have played well and shown off their versatility this year.

• Reuben Foster, the rookie middle linebacker who is a bonafide a game-changer.

Cornerback Dontae Johnson is not one of those foundational pieces.

Johnson, bless him, had another woeful game on Sunday, A week after he was torched by DeAndre Hopkins, Johnson was left spinning by the Titans, who don’t have elite receivers. Rishard Matthews, Corey Davis, and Eric Decker took anything they wanted against the 49ers’ fourth-year corner, who has now gone 54 games without an interception.

It’s fair to say that Johnson would have a hard time making another NFL active roster — even at this late stage in the season — but he was on the field for all 64 snaps against the Titans Sunday.

Opposite Johnson, rookie Ahkello Witherspoon — an impressive physical fit for the Niners’ Cover-3 system — was fine, but Johnson’s poor play required the Colorado product to be near-perfect to cover up for his teammate. He was not perfect, and Marcus Mariota posted a 110.8 quarterback rating for the Titans Sunday.

As a rebuilding team, the 49ers have many serious needs going into this offseason. Luckily, they’ll have ample capital both in free agency and the draft to make those necessary moves.

The 49ers need upgrades at wide receiver, the interior of the offensive line, along the defensive line, and at linebacker, running back, and tight end if they’re to compete for a postseason spot next year, but more than anything they need a cornerback who can shut down one side of the field for their defense.

Without a serious upgrade at cornerback, the entire Niners defense is susceptible — it’s not really a Cover-3 scheme when you always have to bracket the opposing team’s top wide receiver because an outside corner can’t even run with him.

The 49ers’ issue: they need a player that fits a specific role in their defense. Not any cornerback will do.

Cornerbacks in single-high safety systems, need to be big and physical with receivers — Richard Sherman (in his prime) is a good example of the kind of cornerback the 49ers’ should be seeking.

Easy enough, right?

Witherspoon has potential to be that kind of corner, but he’s nowhere near lock-down. And after four seasons, it’s fair to say that Johnson, who is the third-worst rated cornerback in the NFL per Pro Football Focus, is not the guy for the job.

So where do the Niners go to find this all-important piece? They could acquire an established No. 1 cornerback via trade (Sherman might be available) or free agency. If the Niners do opt to add a corner (or two, or three) via free agency, their top target should be the Rams’ Trumaine Johnson, a Cover-3 prototype who has been one of the best cornerbacks in the league for a few years.

That said, Johnson is going to cost serious money this offseason — he’s been franchise tagged by the Rams for the last two years. While the Niners have cash to spend, they might not be keen to put big bucks into the cornerback position — it is a boom-or-bust spot after all.

If the Niners don’t want to use that precious cap space, they could draft a cornerback. The problem with that is that this draft has only one top-10-worthy cornerback — Alabama’s Minkah Fitzpatrick — who while big and physical might be a better option to play as a freelancing safety.

Perhaps the 49ers’ front office just brings in a ton of bargain free agents and third-day draft picks to training camp to see if any of them can stick — that wouldn’t be the worst idea either.

The method, frankly, doesn’t matter. Whatever works. But however the 49ers’ front office does it, they need to have a viable No. 1 cornerback on the field opposite Witherspoon and Williams next season.

On a team with plenty of needs, that role is the most glaring.

And until the 49ers find a fit for that role and fill that hole on the right side of their defense, we can pump the brakes on all of that 2018 hype.