The definition of “hero” should not be “someone who is always there for you,” but rather “someone who is always there for you until they can’t be.” With great baseball players, the moment of letdown results from age. This is what has happened with Bernie Williams.

Yesterday, in what might have been his last game at Yankee Stadium, Williams went 1-for-4 amidst a series of standing ovations and singsong choruses of his name. It would have been fitting had Williams mashed a home run, but the fact that he didn’t is the reason why this day marking the potential separation of Williams and the Yankees became necessary. Old players rarely rise to the occasion.

Williams’s decline phase, which began suddenly in 2003, takes nothing away from one of the great careers in Yankees history. From 1995 through 2002, Williams was a Hall of Fame talent. He batted .321,second in the American League in that period only to Seattle’s Edgar Martinez. With a strong batting eye, he walked over 600 times, leading to an excellent .406 on-base percentage, sixth in the league. Not a pure power hitter, he still smacked 194 home runs, 15th in the league, and slugged .531 (12th). He was third in runs scored and seventh in RBI. He made five All-Star teams, and, though its hard to remember now that his arm is gone (it was never much of an asset in the first place) and his legs have gone, he won four Gold Gloves.

He should have had more. Williams towered over all American League center fielders in the late 1990s. Only Ken Griffey, Jr. hit more home runs. No one hit for a higher average, a greater slugging percentage, took more walks, reached base more often, drove in or scored as many runs. Expanding the picture to include National League center fielders does little to change the picture.

Observers of the time failed to understand just how much of New York’s late 1990s dominance was due to Williams. Center field is primarily a defensive position. Most teams have difficulty finding players who have sufficient ability to play the position and can also carry a bat. Somehow, when it comes to the field of baseball genetics, fielding ability and hitting excellence seem to be mutually exclusive packages. Those players who can do both have received a rare recessive gene. Williams was one of the lucky ones.

In the baseball arms race, this was a huge advantage for the Yankees. Before every game starts, the manager’s lineup card is a set of potentialities. The first baseman has a good chance to hit, the second baseman might or might not, the shortstop probably won’t. The goal is to bring more potential to do damage to the game than does the opposition. It’s more difficult than it sounds. Sluggardly sluggers are plentiful. Every team that does a halfway competent job of assembling talent should be able to put out a first baseman who can hit. Center field is a different story: there just aren’t enough two-way players to go around. If your team has one, chances are the competition can’t match you.

That’s what happened with Williams in his prime. When the Yankees met the Red Sox, Boston could respond to New York’s Tino Martinez with their Mo Vaughn, at worst a push. When it came to Williams, their answer was Darren Lewis. Advantage Yankees, and a big one. The same was true all across the league. With Derek Jeter and Jorge Posada, who also gave the Yankees rare offense at defensive positions, it didn’t matter who the Yankees played at first base, third base, or right field. There were years when the Yankees got subpar production from those positions and won anyway. Williams was a reason why.

Yesterday, some fans carried signs asking for Williams’s return in 2006. They cannot accept that their hero can no longer help them. In baseball, sentimentality is the enemy of winning. Perhaps they’re right to hold on so tightly; it will be a long time before the Yankees again find another player who will have so dramatic an impact on their fortunes.

Mr. Goldman is the author of “Forging Genius,” a biography of Casey Stengel.