Tom Brady understands athletic mortality even as he resists it, as he has created a brand and a life devoted to defeating it.

He proselytizes about muscle pliability, measures the alkalinity of the food he eats and sleeps in special pajamas.

At an age when almost every other quarterback in history had retired, Brady is improving. He plans to play football into his mid-40s. Brady is Ponce de Leon in shoulder pads.

Still, he realizes the reality of professional football. Brady grew up a San Francisco 49ers fan in San Mateo, California, and he was 16 when the 49ers traded Joe Montana to the Kansas City Chiefs. Brady earned his opportunity when the Patriots dismissed Drew Bledsoe, at that point the best quarterback in franchise history. He’s seen Coach Bill Belichick expel some of his best teammates – Lawyer Milloy, Richard Seymour, Willie McGinest, Deion Branch. In the NFL, especially on the Patriots, nobody is indispensable.

“I don’t ever want to play for another coach. I don’t want to play for another owner,” Brady said last winter. “But this is professional sports. I’ve seen some of the best players I’ve ever played with on other teams. I’ve seen Jerry Rice play for the Raiders, Joe Montana play for the Chiefs, Brett Favre play for a lot of teams. You never know. That’s why I want to keep taking care of what I need to take care of. That’s what it comes down to.”

The chances Brady will play the remainder of his career in New England increased Monday night when the Patriots traded his possible replacement, Jimmy Garoppolo, to the 49ers for a second-round draft pick. The Patriots now have no heir for Brady; when they made the trade, they didn’t even have a backup. Brady is 40. He is also, for the Patriots, the future.

Brady staved off Garoppolo, which was never a given. The Patriots drafted Garoppolo in the second round in 2014, when Brady had just finished his age-36 season. “We all know” Brady’s age and contract status, Belichick said. It was a clear message from coach to superstar: We just drafted your replacement.

Brady just would not allow it to happen, with his combination of will, work and skill. If Belichick believed for one second Garoppolo was the better player, Brady would be somewhere other than New England. Brady would not permit it.

But that is not a permanent state. Brady is on track to become the first 41-year-old Week 1 starter in NFL history next season, a year after he became the first 40-year-old. We all still know Brady’s age. Brady has defied age with more elegance and force than perhaps any athlete in history. And yet time will win someday, probably more suddenly than he can see coming.

Belichick is prepared. Even on the day after trading Garoppolo, Belichick reiterated his obligation to evaluate Brady on an ongoing basis. He just traded one possible replacement, which only means the search for the next replacement will begin.

“I’d say when a player gets to a certain point in his career, and that varies from player to player and position to position, but … at some point it becomes year-to-year,” Belichick said. “So the expectations aren’t over a long period of time or longer window like they are with a player coming into the league, where you look at a player’s growth in three to five years or two to four years or that type of a window where a player is young and improving, and physically developing and gaining experience and skill and so forth.

“When you get players that have reached a certain point, then it’s their ability to maintain, although they can work to improve on little things and techniques, and little skills like that. I’m not saying there aren’t things players can do to improve but it’s more of a maintenance and maintaining that high level of play, their maximum level of play, wherever that level is that they’ve reached to sustain that. Trying to predict that is not something – I don’t think it’s easy. It’s not something I try to do a lot of. I look at it as year-to-year. I learned that a long time ago and I’d say that advice has served me well.”

In the immediate aftermath of Garoppolo’s trade, it became easy to think something had changed for Brady and the Patriots. It meant Brady would retire a Patriot, and a charmed career would meet a charmed end, the rarest of all NFL outcomes. In the light of day, after listening to Belichick, it became clear nothing had changed. Brady will be the Patriots’ quarterback until the Patriots can find someone better. Maybe it will be when Brady is 45 and decides he’s had enough. Maybe it will be next season.

It won’t be Garoppolo who replaces Brady. That only means the Patriots will find another quarterback for him to outlast, and another after that, until Brady cannot outlast the opponent he can’t beat.