When Jorge Posada reached an agreement to re-sign with the Yankees earlier this week, it solved one of the major problems of the off-season. There has been a tendency to treat Mariano Rivera’s free agency as a problem requiring the same urgency. It is not. The retention of Rivera might rank high on the sentimentality meter, but as far as impact on the bottom line of winning and losing games, it won’t have nearly the same impact as the return of Posada.

Posada is one of the best offensive catchers of the era. Compare him to Ivan Rodriguez, who will someday waltz into the Hall of Fame, and you’ll see that all the more heralded Pudge has on Jorge is career length and batting average. Since Posada became a regular, he’s outhit the perennial All-Star, and the gap between them has widened greatly. Had Posada departed, the Yankees would have had no chance at maintaining offense at the position, a huge blow given that Posada’s hitting cushions the Yankees from the full impact of playing weak bats at first base and center field. Posada isn’t just a catcher; he’s a subsidy.

It’s tempting to look at Rivera the same way. He’s an instant Hall of Famer, one of the all-time great relievers, or at least he had been before he slipped a bit last year. But Rivera’s impact is limited by the small amount of time he spends on the field. Rivera threw approximately 5% of the team’s innings in 2007. Even though the Yankees used him in many crucial situations that magnified the importance of those 71 innings, the fact is that his opportunities are limited.

This is due in large part to the way closers are used. Managers long ago conflated what the saves rule defines as a safe situation and actual turning points in a game, which may come at any time before the ninth inning. As recently as 30 years ago, top relievers like Goose Gossage would pitch 100 or more innings a year (Gossage did it four times), intervening to defuse emergencies anywhere between the second and the ninth. They often pitched to preserve a lead, just as they do today, but they also were used to hold the line in a tie or keep a losing score close enough for the offense to come back.

While the effects of the heavier workload are still debated, it is clear that this was a more superior usage pattern than today’s, when many teams will save their best relievers to preserve a three-run lead in the ninth, which very, very few pitchers will blow with any consistency, and let vastly inferior pitchers toil before then, which often has the effect of turning a small lead into a loss, or a small deficit into a large one.

To put it another way, there is a big difference between using a Mariano Rivera or a Brian Bruney in the sixth inning of a tie game, but much less of a difference between using Rivera or Bruney with a three-run lead in the ninth. Rivera’s career regular season save percentage is 88% — 443 saves, 59 blown saves. The deservedly derided Joe Borowski of the Indians saved 45 games this year and blew eight, a conversion rate of 85%. Bobby Jenks, hardly America’s most reliable reliever, saved 40 of 46 chances, or 87%. Todd Jones, who left his good stuff in the previous millennium, saved 38 of 44 chances, or 83%.

To most teams these differences are not worth millions of dollars, much less the $15 million a year for three seasons the Yankees have offered Rivera. The proposed deal would make him the highest-paid reliever and the fourth highest paid pitcher in baseball. Among relievers he would best even Billy Wagner, whose four-year, $43 million contract pays him $10.5 million a season, and B.J. Ryan, who two years ago signed a five year, $47 million contract that was at the time the largest for any reliever. He’ll make just $10 million a year over the next three seasons.

The reason that a Hall of Famer like Rivera’s results aren’t too different from that of a Borowski or Jones is simple. It’s that saves rule again and the way baseball is set up for failure. Imagine the worst possible pitcher going out to protect a three-run lead. Even if opponents average .400 against him, he still has a 60% chance of retiring each batter, and on most days he’ll be able to get three outs before he gives up three home runs. When Rivera goes to the Hall of Fame, his postseason numbers will be lauded, but he’ll deserve even greater plaudits for his consistency. Most relievers yo-yo between good years and bad; even the good ones do so, albeit in a narrower range. Rivera has not, at least not until 2007, when he posted the highest ERA of his relief career — and still converted the same 88% of his saves chances (again, saves aren’t about the quality of the reliever; they’re about the rule). Rivera will celebrate his 38th birthday a week after Thanksgiving. In the coming seasons he’ll pitch less, not more, and consistency will be even more of a challenge.

Rivera has meant a great deal to the Yankees franchise and to Yankees fans, but given his age, usage, and the borderline-rebuilding phase the team is in, the money they spend to retain their closer will have more to do with past glories than his impact on future championships.

Mr. Goldman writes the Pinstriped Bible for yesnetwork.com and is the author of “Forging Genius,” a biography of Casey Stengel.