The Boston Celtics signed the NBA’s top free agent big man on Saturday.

The Kevin Garnett era could have ended with his retirement, but instead it continues for three years valued at a reported $34 million. Depending on the amount of guaranteed seasons on Garnett’s new contract, the Celtics could pay him more than $11 million in 2014-15 when he will be 39 years old. The prospect sounds murky until one remembers just how magnificently Garnett played from the All-Star break through Game 7 against the Miami Heat, and the fact that he’s one of the NBA’s best-conditioned athletes on a yearly basis. It’s still a risk given how many miles for which Garnett has ridden during his career, but Garnett is one of few players who could possibly deserve being paid $11 million to play basketball one year before age 40. Seriously, look at pictures of Da Kid as a youngster with the Minnesota Timberwolves and as a 36-year old with the Boston Celtics. He has added a few pounds of muscle since his rookie year, but his body has changed a ridiculously small amount considering he’s been in the league for 17 seasons.

In a vacuum, the Garnett signing is great. He was the single best big man in the NBA playoffs and he’s returning to the Celtics for three years at DeAndre Jordan money. There’s sure to be at least some level of slippage between now and the time Garnett’s new deal expires, but we can be sure he’s fully dedicated to maximizing the final few years of his career, just as he has been fully dedicated to maximizing every year of his career. He also doesn’t seem like the type to hang on past the time when he feels like he can still contribute to his expectations. If Garnett stops playing at an All-Star level at any point during his three-year deal, he’ll probably just retire.

Garnett’s the best sensei Jared Sullinger and Fab Melo could ever hope for (though Patrick O’Bryant raises his hand to politely remind us that he doesn’t have a 100 percent success rate with his young projects), and — judging by his absurd playoff run (not to mention his entire Hall of Fame career) — he’s far more than that. He’s still one of the NBA’s five best defenders, one of its top midrange jump shooters and he can still succeed in the post especially when he has the right matchup. According to Basketball Value, the Celtics were 35.1 points per 100 possessions better with KG on the court during the NBA playoffs. Losing him would have been devastating to Boston’s chances of fielding a roster than can compete for anything worthwhile. Keeping him almost definitely ensures that the Celtics will take another shot at a championship, regardless of how successful you or I think that shot might be.

The Celtics’ rebuilding process still hasn’t begun. Some people would argue they should have cut ties with Garnett, traded Paul Pierce (and the last two years of his contract) for some young pieces, freed all their potential cap space and started to build around Rajon Rondo. That perspective has some merit. But with the free agency class presenting slim pickings, the “rebuild now” path would have led the Celtics to at least one or two seasons of mediocrity and irrelevance. Regardless of how you feel about Boston’s chances next season, the Celtics are now far from irrelevant. The same team that came within 10 minutes of reaching the NBA Finals (and gave Miami significantly more fits than any other squad) might return next season noticeably better, at least from a talent standpoint.

The Celtics have a core of Garnett, Pierce, Rondo and Avery Bradley already in place. Jared Sullinger and/or Fab Melo might contribute (Sullinger’s the better bet). The C’s still have the rights to several free agents, including Ray Allen, Jeff Green and Brandon Bass, who would all fetch interest in sign-and-trade deals if the Celtics can’t (or decide not to) retain them. And the Celtics also have the option of renouncing the rights to their free agents (including the aforementioned trio, Greg Stiemsma and Nenad Krstic, to whom they still own Bird rights) and opening $13 million in cap room to be used how they see fit. Put more simply, Danny Ainge has assets and flexibility like he didn’t have last offseason, when he still managed to build a team that overcame four season-ending surgeries to reach Game 7 of the Eastern Conference Finals.

John Hollinger suggests that the Celtics might re-sign Green and Bass to market-value deals, match Stiemsma’s offer sheet (if any team extends him one) and still have a low enough payroll to make a serious run at a quality free agent (maybe O.J. Mayo) using the full mid-level exception. The Celtics could also re-sign Green, Bass AND Allen (which would likely take away their opportunity to use the full mid-level) and still have the mini mid-level to sign a veteran who would fit nicely. That would give the Celtics a top 10 of Rondo, Bradley, Pierce, Bass, Garnett, Allen, Green, Stiemsma, Sullinger and MMLE Free Agent X. Impressive, at least on paper. And remember, the C’s almost qualified for the Finals last season with a disabled Ray Allen in the starting lineup and a bench that mustered just one point combined in the final game of Boston’s season.

Aging and injuries could always arise as issues (although Garnett seemed to age backwards last season) and Derrick Rose will return to the Chicago Bulls at some point next season, so the Celtics are far from a lock to be one of the final four teams standing in the NBA playoffs again next season. But with Garnett’s return, we are assured the Celtics’ goals are again focused on the now as well as the later. Ainge will surely look to remain cap flexibility for the future (look for most of Garnett’s third year to be non-guaranteed so the Celtics can clear cap space for the summer of 2014 when Pierce’s contract expires), but he’s also interested in shaping a roster that can compete to knock the Heat from their current perch as Eastern Conference (and NBA) champions.

The Celtics’ three-year window has already become a five-year window, and Garnett’s new deal threatens to keep that window cracked for at least another year or two.

At the very least, that’s Ainge’s plan.