New circumstances can breed new confidence. When the two combine, a baseball player can be transformed so radically he hardly recognizes himself.

For those who watched John Tudor as a member of the Boston Red Sox and Pittsburgh Pirates, the emergence this season of the 31-year-old left-hander as one of the game's best pitchers is barely conceivable.

In fact, one of those most familiar with the old Tudor is the new Tudor. "It's beyond my imagination how I can continue to do it," he said.

After starting the season 1-7, Tudor has, in his last 24 starts, gone 19-1, pitched 10 shutouts and lowered his ERA from 3.34 to 2.04. He's won his last 10 decisions in a row, has four shutouts this month and recently reeled off 31 2/3 consecutive shutout innings. Two weeks ago, he beat the New York Mets, 1-0, in 10 innings after matching zeroes with Dwight Gooden for nine innings.

Tudor's manager, Whitey Herzog, said, "Call him the best left-hander in the game." That leaves comparison with Gooden, a right-hander, to others.

Such a season, such a streak, would be the stuff of fantasy for a great pitcher. For a man who was 34-28 in the minors and 52-50 in the majors when all this foolishness began, for a fellow who never won more than 13 games in a season anywhere and has no eye-popping pitch, such things are hallucinatory.

They also cry out for explanation.

Tudor has no new pitch, guru, religion or training program. He hasn't discovered control, changed his windup, quit smoking or gotten a mantra.

"I haven't changed," he said.

What we have is an almost textbook example of a pitcher who has benefited from almost all of the basic changes in circumstance that historically have improved pitchers in midcareer.

What do .500 pitchers dream about on long winter nights?

*Playing for a high-scoring, first-place team.

The Cardinals, with the best winning percentage in baseball (.634), lead the National League in scoring, with 50 more runs than any other club and 200 more than the Pirates, who dealt Tudor away for George Hendrick (.235).

*Going from a small park to a huge one.

In five Fenway Park seasons, Tudor was a prototypical southpaw bedeviled by cheap home runs. Even at spacious Three Rivers Stadium last year, he ranked a lousy 93rd in the league in percentage of home runs allowed. Just as interesting, Tudor has always "thrown fly balls": a career ground outs-to-air outs ratio of 0.791. He seldom gets a double play grounder (one every 30 innings in 1984).

Changing from a slow defensive team to a wonderfully swift one.

If anybody ever needed a huge turf park with fleet outfielders to run down fly balls, it was Tudor. In Boston, he had slow defenders. Now, he has perhaps the best ground-gobbling team in history behind him. "A godsend," Tudor calls his defense.

Herzog flatly says he'd take his three outfielders as a group -- Vince Coleman, Willie McGee and Andy Van Slyke -- "over any other three in the whole league." As for his infield, "it would be close," according to Herzog as to whether they might not be as good as an all-star glove team from the rest of the league.

Superb defense has turned Tudor's control from good to great. "When I get behind in the count, I think, 'I'm a fool if I walk this guy when Willie McGee might run down a gapper.' "

In the past, Tudor has not pitched well with men on base or in pressure situations. In 1984, for instance, his 3.27 ERA was fine, but the league hit an amazing .390 in pressure situations against him; only three of 123 pitchers in the NL were worse. Now, with fewer walks and fewer hits allowed, Tudor faces fewer of the situations that bothered him.

As another bonus, Tudor went from the home run crazy American League, where hitters loved his fly-ball style, to the speed-daffy NL, where turf-style hitters hate a lefty who makes them hit in the air and can hold them close to base.

Finally, the Cardinals' no-name bullpen has been wonderful all year.

When did one pitcher ever have such a confluence of subtle factors all working in his favor at once?

Even a streak like Tudor's doesn't make a man immune to worry. Thursday night, Tudor was concerned that, in each of his two previous starts, he'd given up a grand slam -- that old bugaboo of men on base, poor pressure pitching and gopher balls was rearing its head.

Fortunately, a cold night and a disinterested Phillies club helped Tudor get in a groove and ring up yet another shutout.

How Tudor will cope with late-season and postseason pressure should be a nice question for baseballologists. His raw stuff is unprepossessing -- just a pretty good fast ball, a servicable curve ball of varying speeds and a fine changeup that is idiosyncratic because sidearmers seldom use such a pitch.

"Reminds me of Larry Gura," said Herzog. "Gets the fast ball amd slider in on the fists and breaks a lot of bats because guys are leaning over the plate to get to his slow stuff."

Gura never went 19-1.

As a psychological buffer, Tudor has hidden within the team's success. He's maniacal about sharing credit -- the hallmark of a man having a once-in-a-lifetime season.

Most consistently great pitchers have bearing and presence. Tudor does not, at least yet. Some guys are hard to read. Not Tudor. Everything about him says, "Peabody, Mass." That's pronounced "Pea-biddy." He's working-class Irish. You'd guess Boston-area cops were in his family tree even before you learn that his degree from Georgia Southern was in criminal justice.

Tudor's not charming, not open, not polished with strangers. He saves his smiles, doesn't mind giving an abrupt or dismissive answer. The chip on the shoulder, the kind that comes from playing hockey at Peabody High, not Groton, hasn't gone away. Tudor has something to prove.

He's not big -- 6 feet, 185 pounds. And he's just a regular-looking fellow, handsome straight ahead but weak-chinned in profile. Playing the star is impossible for him. He casts his eyes down during interviews, chewing his own private joke as he listens to questions usually reserved for glamor boys.

Nobody ever gave him the star treatment. He didn't turn pro until he was 22 and never led any league, even the minors, in anything.

Nothing in sports is rarer than an athlete who exceeds his own fantasies, his own sense of his limits. Slowly, the national spotlight is finding this modest man. The circumstances for a Tudor reign are still aligned. Now, what remains is the difficult job of holding onto that new and elusive confidence.