SANTA CLARA, Calif. -- From the moment the San Francisco 49ers used the No. 2 overall pick in the 2019 NFL draft on him, defensive end Nick Bosa has been one of the franchise's most important players.

In the more than four years since, the only thing that's changed is that Bosa has somehow managed to become even more important to the Niners' success. Any remaining doubt about just how much Bosa means to the 49ers vanished Wednesday, when he agreed to a five-year, $170 million contract that includes $122.5 million in guarantees, sources told ESPN's Adam Schefter. The deal makes Bosa the highest-paid non-quarterback in NFL history.

Getting Bosa back in the fold following a 43-day holdout that lasted longer than many expected was imperative for the 2023 49ers. Bosa is the reigning NFL Defensive Player of the Year, and there's a strong argument that he's the player the Niners can least afford to lose. With Bosa, the Niners' Super Bowl aspirations are legitimate. Without him, well, it's far more difficult to envision.

Since Bosa entered the league in 2019, San Francisco is 43-17 in games he has played and 5-10 in those he hasn't, including the playoffs. In that same time frame, the 49ers have the highest pressure rate in the league (32%) when Bosa is on the field and the second lowest (25%) when he's not. He's expected to be in uniform Sunday against the Pittsburgh Steelers (1 p.m. ET, Acrisure Stadium, Fox)

Bosa's value isn't just a short-term thing. Now that he's under contract through the 2028 season, Bosa is not only the team's most dominant defender, he's also the player charged with bridging the gap between San Francisco's current nucleus of stars and a young, emerging group that is aiming to keep their championship window open well beyond this season.

"I think everyone's a product of their environment, so everyone thinks they are based off of people they're around," coach Kyle Shanahan said. "Then, when you get around different people or you think you're one way or another and then ... you see how dedicated Bosa is to his craft, not just here but year-round.

"And I think that's just what makes everyone change. Everyone has a certain standard and then when you see other people at a higher standard who also play at a higher level, that kind of brings everybody up and shows you what it takes to be this good in this league."

In just 51 regular-season games, Bosa has amassed 43 sacks, 156 tackles, 56 tackles for loss and 106 quarterback hits on his way to that DPOY, Defensive Rookie of the Year, three Pro Bowls and a first-team All Pro nod. The frightening reality for opposing quarterbacks and offensive linemen? Bosa won't turn 26 until Oct. 23.

Which means Bosa is just entering his prime. Teammates and coaches constantly rave about his discipline and work ethic, which is why they have little doubt he will return to the team in shape and ready to make an impact immediately. It's also why they think Bosa can be even better coming off a career year.

"I guarantee you he probably will say that what he did last year wasn't enough," left tackle Trent Williams said of Bosa, who had 18.5 sacks in 2022. "I guarantee you he'd say he left a few sacks out there on the field and I guarantee you his main focus is to take everything off the plate and not leave a crumb. ... Nick being the guy he is and being how he got this good is not because he rests on his laurels. He's always his worst critic and he continues to care about getting better. Being that he's 25 only makes it scarier for guys."

Bosa's relative youth also makes him and linebacker Fred Warner, who will turn 27 on Nov. 19, the team's two most prominent younger players under long-term contracts. Whether the 49ers sustain success for years to come will depend on whether Bosa and Warner can continue to produce at elite levels on the field while providing the blueprint for young Niners entering the building.

Veterans such as Williams (35), defensive tackles Javon Hargrave (30) and Arik Armstead (29), tight end George Kittle (29), fullback Kyle Juszczyk (32), receiver Deebo Samuel (27) and running back Christian McCaffrey (27) all have plenty of good football in front of them, but they've likely already signed the biggest contracts they'll get in their career, at least in San Francisco.

At some point, perhaps in the next couple of years, the Niners will have to reallocate those resources to some of their emerging young stars. Players such as safety Talanoa Hufanga (23), linebacker Dre Greenlaw (26), wideout Brandon Aiyuk (25) and, most important, quarterback Brock Purdy (23) are among the names the Niners would like to see grow into bigger roles, both in terms of production and in the locker room pecking order.

And it's not inconceivable that Bosa has another big deal left in him before his time with the 49ers is done, as his team could start negotiating his next extension when he's 29 or 30.

Aiyuk is eligible for an extension for the first time after this season. The significance of Bosa's contract wasn't lost on him.

"We are all playing this game, we all know what comes with the game when you play the game at a high level, you get to reap the benefits like he did," Aiyuk said.

Upon his return to the 49ers after nearly six weeks away and the largest contract in franchise history in tow, Bosa won't be asked to do anything different. The emotional and vocal leadership responsibilities still belong to Warner.

But Bosa sets the example for all of his teammates every day, and now, for the next six seasons, they can see what it takes to not only keep the Niners in contention but secure the financial future for their families.

"That's the goal of how you want to do it," Shanahan said. "You want your players to see that. You hope you get the guys who play the right way and at a certain level and you can reward them."