The San Francisco 49ers started using the No. Nick Bosa, a defensive end, is the No. 2 overall pick in the 2019 NFL draft. He has been one of the most important players for the team.

The only thing that has changed in the more than four years that have passed is that Bosa has somehow managed to become even more crucial to the Niners’ success. Sources told ESPN’s Adam Schefter that Bosa agreed to a five-year, $170 million contract with $122.5 million in guarantees on Wednesday. This eliminated any lingering doubt about how much Bosa means to the 49ers. Bosa becomes the NFL’s highest-paid non-quarterback as a result of the deal.

The 2023 49ers had to get Bosa back into the fold after a 43-day holdout that went longer than most people expected. There is a strong argument that the Niners can least afford to lose Bosa, who is currently the NFL Defensive Player of the Year. The Niners’ aspirations of winning the Super Bowl are real with Bosa. Well, it’s much more difficult to imagine without him.

San Francisco is 43-17 in Bosa’s games played and 5-10 in Bosa’s games not played, including the playoffs, since Bosa joined the league in 2019. When Bosa is on the field, the 49ers have the league’s highest pressure rate (32%), but the second-lowest (25%) when he is not. Bosa’s value isn’t just short-term; he is expected to play Sunday against the Pittsburgh Steelers (1 p.m. ET, Acrisure Stadium, Fox). Bosa is the team’s most dominant defender now that he is under contract through the 2028 season. He also has to bridge the gap between San Francisco’s current star-studded roster and a young, up-and-coming group that wants 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.”

Bosa has won the Defensive Rookie of the Year award, three Pro Bowls, and a first-team All Pro honor in just 51 regular-season games. Along the way, he has recorded 43 sacks, 156 tackles, 56 tackles for loss, and 106 hits on the quarterback. What is the terrifying reality for quarterbacks and offensive linemen on the other team? On October 23, Bosa will turn 26.

This indicates that Bosa is still in his infancy. His teammates and coaches frequently praise his work ethic and discipline, so they are confident that he will immediately return to the team in good shape. Additionally, this is why they believe Bosa can improve even further after 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.”

Due to Bosa’s relative youth, the team’s two most prominent younger players on long-term contracts are also linebacker Fred Warner, who turns 27 on Nov. 19. If Bosa and Warner are able to continue producing at the highest possible levels on the field while serving as a model for the upcoming generation of Niners, it will determine whether the 49ers will continue to be successful for many years to come.

Veteran players like Williams, who is 35 years old, 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 a lot of good football to play, but they’ve probably already signed the biggest contracts they’ll ever get, at least in San Francisco.

The Niners will have to redistribute those resources to some of their upcoming young stars at some point, possibly in the coming years. The Niners would like to see players like safety Talanoa Hufanga (age 23), linebacker Dre Greenlaw (age 26), wide receiver Brandon Aiyuk (age 25), and, most importantly, quarterback Brock Purdy (age 23) advance into larger roles, both in terms of production and in the locker room hierarchy.

Bosa’s team could begin negotiating his next extension when he is 29 or 30, so it is not out of the question that he will have another significant contract before his time with the 49ers is up.

After this season, Aiyuk will be eligible for a contract extension for the first time. He was well aware of the significance of Bosa’s contract.

Aiyuk stated, “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,”

Bosa won’t be asked to do anything different when he comes back to the 49ers after nearly six weeks away and with the biggest contract in franchise history. Warner still bears the emotional and public leadership responsibilities.

But every day, Bosa sets an example for his teammates, and for the next six seasons, they can see what it takes to keep the Niners in contention and ensure their families’ financial futures.

“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.”