Randy Johnson is looking for the soft landing. An aging flamethrower who senses his torch guttering out has two choices: He can reinvent himself as a precision pitcher, shifting his repertoire to accommodate his diminished stuff, or he can try to go about things as he always has, rage against the dying of the light, and ultimately make as effective a last stand as did Custer or the dinosaurs.

Before we critique Johnson’s position as one of advanced age and uncertainty, though, we should whisper one other word that might account for his uncharacteristic loss of location: injury. Although no specific information has emerged to support the theory, the Yankees sent the Big Unit in for an MRI yesterday just to make sure he wasn’t hurt. Unlike, say, the Mets’ Victor Zambrano, who knew his arm was hanging by a thread but hewed to a “don’t ask, don’t tell” policy, Johnson may not yet even know that he’s compensating for something.

Put that aside for now. Keep it in the back of your mind as a possible secondary or tertiary cause of Johnson’s inconsistency, because the simplest and best explanation may simply be age. Johnson is 42. Very few pitchers maintain their stuff until 35,let alone 42.Very few pitchers have been truly dominant at 40 or older. Roger Clemens, 42 last year, and Johnson himself as a 40-year-old in 2004 had Hall of Fame-quality seasons that far outstrip anything accomplished by their aged peers. The list of quatrogenarians that were nearly as good is dominated by spitballers like Jack Quinn and knuckleballers like Phil Niekro, Charlie Hough, and Hoyt Wilhelm. The suggestion would seem to be that when a hard thrower reaches a certain age, he’s gone without a second act.

Historical analogues are only moderately useful when it comes to Johnson, who’s a unique specimen. There haven’t been many 6-foot-10-inch lefties with 4,000 strikeouts in major league history. Still, we know that even the greatest pitcher is subject to limits imposed by age, and at some point all the adjustments in the world won’t help him find his former dominance, or even some fraction of it. That’s when it’s time to move on, collect your Hall of Fame plaque, do some baseball card shows.

Consider two of the best fastball-slider southpaws of the 1970s: current Yankees pitching coach Ron Guidry and Hall of Famer Steve Carlton. Guidry had his last good season at 34 and was done at 37. Carlton saw his last great season arrive at age 36, his last significantly above-average season at 38, and though he hung on until he was 43, his ERA was more than a run above league average.

These were intelligent pitchers, crafty as well as dominating, yet at some point age and injury made adjusting beside the point. Tom Seaver, one of the most intelligent pitchers of all time, hung up his spikes after the 1986 season. He was 41. The next year he had second thoughts, and when every Mets pitcher with a vowel in his name went down due to injury or incapacitation, Tom Terrific agreed to a last hurrah in orange and blue. But as much as the Mets, Seaver, and the fans would have loved to see the reunion, when it came time to limber up and throw, the pitcher found that his 42-year old body just couldn’t hold up its end of the bargain. He returned to retirement without taking the mound.

Johnson’s performance last season would seem to argue against his having reached this kind of sudden end. The Big Unit struck out 211 batters, the seventh-most ever by a pitcher 40 or older in a single season (the list is dominated by Clemens, Nolan Ryan, and Johnson himself). His career strikeout rate of 10.95 batters per nine innings dipped to 8.42, which would have been more troubling if he hadn’t transferred to the DH league, where you don’t get to strike out the pitcher a minimum of two times a game. Yet there were other signs, particularly the frequently flat slider that batters blasted over the wall a career-worst 32 times. Resultantly, Johnson’s equivalent earned run average (ERA) of 3.93 was his least impressive of any non-injured season since 1992.

This season the signs are more troubling. Even as Johnson struggled with his slider last year, his control remained impeccable. Like many young lefties, Johnson was extremely wild in his 20s, walking somewhere between five and seven batters per nine innings from 1989 through 1992. The following season, when Johnson was 29, he literally got things under control – his walk rate dropped from 6.16 per nine to 3.49, and, with occasional fluctuations, continued falling through last season.

This is one of the most amazing aspects of Johnson’s career; some pitchers struggle with wildness and overcome it, but few have throttled it into total surrender as Johnson has. In 2004, his walk rate dropped under 2.00 for the first time, falling to 1.61. Last season it was 1.87, a rate which the Twins’ Brad Radke, who probably has the best control in the majors, wouldn’t sniff at.

As Tuesday’s start against the Red Sox demonstrated, those days are, at least momentarily, gone for Johnson. It wasn’t so much the seven runs (only two were earned) and five hits he allowed to the slugging Sox as much as the five walks and two poorly timed wild pitches. Overall, his walk rate is still a respectable 2.69 per nine innings, but in his last three starts, that rate has shot up to 6.46.At the same time, his strikeout rate has drooped to an un-Unitonian 5.36.

This last may simply be a sign of hitters recognizing that as long as Johnson is unable to locate his slider, a fat fastball will be coming their way. Whatever the state of Johnson’s velocity, a big league hitter can hit any pitcher’s fastball if he knows it’s coming.

Taken together, the rising walk rate and declining strike out rate are more alarming than either would be separately. Whether Johnson’s problem is age, injury, or both, they signal that something could be significantly wrong, something that may not be easily fixed.

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