SANTA CLARA, Calif. -- In an offseason full of uncertainty for the San Francisco 49ers, one thing that's known for sure is that their top priority is retaining star left tackle Trent Williams.

The Niners want Williams. And Williams, who is set to become an unrestricted free agent on March 17, has made it clear he'd like to stay. Seems easy enough, right?

As is often the case when it comes to matters of multimillions, it's not so simple. With a chance to see just how much he could garner on the open market for the first time in his career, Williams is likely to test the waters.

"San Francisco would be the leading candidate regardless," Williams said in January. "So even if it does make it to free agency, it won't be because I'm trying to go somewhere else. It may be simply because I want to see my value. It's been 11 years in this league, and I have yet to see a franchise left tackle go to the open market. I think it would be interesting to kind of see what [my] value holds."

Nothing has worked out just yet between the Niners and Williams, but not for a lack of trying. Before the end of last season, the Niners and Williams had discussions about a long-term contract. No deal was reached then, nor has one been struck since.

Complicating matters further, the 49ers don't have the option of tagging Williams after adding a clause to his reworked contract forbidding it after his trade to San Francisco was made official. On top of that, the NFL salary cap is expected to dip into the $180-185 million range, leaving the 49ers with a projected $13-18 million in cap space before making any cost-cutting moves.

All of which means there's an increasingly real chance that Williams will indeed get to at least find out what teams are willing to offer for his services.

"I think it's pretty unusual for a guy like that [to hit the open market]," said Mike Tannenbaum, ESPN's NFL front-office insider.

Tannenbaum cites Branden Albert, Nate Solder and, if you remove the designation of right or left, Jack Conklin as semi-recent comparisons who have become unrestricted free agents. But none have Williams' résumé. He's an eight-time Pro Bowl selection and reaffirmed that standing with a dominant 2020 season after missing the 2019 season because of a rare form of cancer and a dispute over the handling of his health and his contract status with the Washington Football Team.

"Trent has been an awesome teammate, an awesome competitor and obviously one of the elite offensive tackles in football," right tackle Mike McGlinchey said. "The job that he's done for us this year speaks volumes, and hope to God we can keep him for a little bit longer here."

Williams is the focal point of a 49ers offseason that can go in many different directions.

To keep him, the 49ers will probably have to pay Williams in line with the likes of premier left tackles Laremy Tunsil, Ronnie Stanley and David Bakhtiari. Each has signed a new deal averaging from $19.75 million (Stanley) to $23 million (Bakhtiari) per season in the past year.

Williams, 33, is older than each of the five highest-paid left tackles in the game by at least three years. That and his injury history could keep his cost down a little, but that is likely negated by the fact that he's an elite player at a premium position poised to be on the open market.

"How I would normalize ability and age, I would enter the average per year he deserves like Bakhtiari, Tunsil, [and] it has to be I would think at least $20 [million] a year," Tannenbaum said. "Then, from a guarantee standpoint, he may have to take a little bit more of the risk because of the age, but if he finishes the season, he certainly should be compensated consistent with those players."

Undoubtedly, those are some big numbers for a team that isn't flush with cap space and will be bidding against teams that can more easily afford such a lofty price tag. Teams such as the Jacksonville Jaguars and Indianapolis Colts have a glaring need at the position and more than enough space to throw lucrative offers at Williams, who has also said he's not interested in going to a rebuilding situation.

Could Williams accept a bit less to stay with the Niners should another team offer more money?

"I don't know if 'discount' would be the right word, but as far as the payout, you can compromise with that," Williams said. "There are so many different ways to make a contract fit."

One possibility is to follow a similar track to what the 49ers did with defensive lineman Arik Armstead last year. Armstead was a free-agent priority, and the Niners had a similar amount of cap space available going into that offseason as they will have this year. Armstead hit the open market, but the Niners eventually closed the deal after Armstead was able to field offers during the legal negotiating window.

What's more, the 49ers structured Armstead's deal to limit his cap numbers to $6 million in 2020 and $12.5 million in 2021 on a contract that averages $17 million annually. They did that by giving him a $25 million signing bonus that is paid upfront but spread over five years on the cap.

Something similar with Williams, where he gets a sizable signing bonus along with guaranteed salaries in the first season or two, would make sense before larger cap numbers in 2023 and beyond when the cap is expected to return to closer to normal.

If the 49ers are unable to keep Williams, it could create a significant shift in the offseason plan. Instead of a big payout to Williams, that money could be spread around to other positions, possibly including a cheaper left tackle replacement. More likely, left tackle would become the team's top need in the NFL draft, and it could put the 49ers in the market for a pricier edge rusher to complement Nick Bosa.

And if Williams does stay? It likely means the Niners will have to say goodbye to some of their own free agents they'd prefer to keep.

"It always comes down to allocation," Tannenbaum said. "So basically what you want to say is, 'Hey, look, if we stretch on Trent, here's what we can't do.' I think they've done a pretty good job of that. Like, they said, 'We can't pay DeForest Buckner, so we're going to have to draft Javon Kinlaw.' Those are sort of the things you have to look at, so if you feel like you have to have Trent, then here's what we can't do."