In baseball, the half-life of a myth is measured in years. It takes time for reality to catch up with legend. In recent years, two of baseball's most firmly entrenched opinions have been that, say what you might about George Steinbrenner and Billy Martin, the big cheese eater knew how to build a team and the little one knew how to manage it.

That may no longer be the case.

The epitaph of the 1983 New York Yankees may read thus: few teams have ever amounted to so much less than the sum of their parts.

The verdict on Billy Martin's work this season may be equally trenchant: Seldom has a celebrated tactician had such a bad year calling the shots.

If ever a baseball team has been misassembled by the front office (i.e., Steinbrenner), then mismanaged in the dugout, it is these slumping fifth-place Yankees who find themselves 3 1/2 games out of first place after losing six of their last eight games.

The rest of the AL East thinks the Yankees have more talent than any other club.

"What do the Yankees need?" asks Detroit's Sparky Anderson. "They don't have no needs. I looked over there and saw Roy Smalley, Oscar Gamble, Lou Piniella, Omar Moreno, Don Mattingly and Butch Wynegar. And that was their bench.

"Steve Kemp was batting eighth for 'em and he was my No. 3 hitter here. They've got the most talent in this league."

"The Yankees will win the East," said Cleveland's Toby Harrah. "Wait'll you see their 40-man roster in September. They'll have enough guys out there for a football team. They'll win by 5-6-7 games."

Between illusion and reality falls the shadow, it's been said. That's true of baseball, too. For instance, it's part illusion that the Yankees have great talent. They're overloaded at a few positions (outfield, first base and designated hitter) with a stockpile of good, but not great, players. The Yankees can field endless varieties of solid, but not excellent, combinations at these spots; indeed, they have presented a kaleidoscope of capriciously chosen lineups all season.

"Ralph Houk used to say, 'Names don't win games,' " said Yankees Coach Roy White on Thursday. "Back in '77 and '78, when we were world champions, our names never equaled the Red Sox names, but we knew our jobs and complemented each other.

"Look at Toronto. On paper, they can't touch us. But on the field, they do the right things--get the bunt down, throw to the right base," continued White. "We've won when we've blown people out, but we haven't been good in close games (12-15). We have not made the important little plays all year."

The Blue Jays have outscored the Yankees this year. New York has the "name" catchers--Wynegar and Rick Cerone, both of whom want to start. But the contented Toronto catchers, Ernie Whitt and Buck Martinez, have nine more homers and 19 more RBI. Toronto's who-are-they third basemen--Rance Mullicks and Garth Iorg--are both hitting .285. By contrast, the millionaire free agent Kemp, upset by being benched, is hitting .251.

Yankee pitching is also part smoke. No contender has starters as weak as Matt Keough (5.40 ERA), Jay Howell (5.19) and $2 million free agent Bob Shirley (5.49); this trio has started 33 games already. "We've never found a right-handed starter," says White.

Often overlooked is the Yankees' weakness up the middle. The team is 12th in fielding with 99 errors. Wynegar is only adequate. Willie Randolph has missed 56 games with injuries. Shortstop Andre Robertson didn't start until May and is now on the disabled list because of injuries received in a car accident this week. Jerry Mumphrey was so overmatched by Yankee Stadium's center field that New York traded him last week for Moreno.

These Yankees have been collected more than assembled. The novice Yankees' front office--few career baseball people will work for Steinbrenner--appears to be underqualified and inexperienced. Who knows Murray Cook, the Yankees general manager?

Compounding the panic is Steinbrenner's inability to resist grabbing any player whose name has ever been mentioned by Howard Cosell. Insecurity dominates the New York clubhouse.

However, the Yankees have another liability.

Baseball mythology says that Martin is a marvelous manager. Therefore, he must be doing a decent job this season. Actually, he's managed poorly this year--long on hunch and bile, short on logic. Martin has been manager of the year in four of the last nine seasons and he deserved the awards. But, this year, he may be the worst manager in the AL East.

Martin's worst sin is his callous abuse of Ron Guidry's arm; what he's done to Louisiana Lightning would be a horse-whipping offense in some states. Guidry is being Martinized, like Steve McCatty, Mike Norris, Rick Langford and so many others before him.

Entering '83, Guidry--with the second-best winning percentage in baseball history--had pitched 11 complete games in 83 starts in the '80s. This year, Guidry, who needs to conserve the sliders left in his skinny arm if he wants to reach Cooperstown, has completed 14 of 23 starts. He's allowed 13 and 14 hits in games, thrown 140 and more pitches and generally been treated like a plow horse, not a thoroughbred. Guidry, who had been out earlier for three weeks with a bad arm, has slumped badly in the last two months.

Goose Gossage, also overused in Martin's binges, has lost something off his fast ball and has squandered leads all month; like Guidry, he's 32 and needs care. Martin has castigated starter Shane Rawley (11-10) for "not being hungry enough," when, in fact, Rawley may merely be feeling the effects of the first season when he's ever pitched more than 165 innings.

Martin's ingrained penchant for alibiing has now become constant. "When we lose, we always blame something; it's never our fault," says one Yankees front office official. On Thursday, Martin said of Lee MacPhail, American League president, "I don't know who his vendetta is against." Martin has called one umpire "a stone liar," and intimated all year that the Yankees were the victims of umpire bias.

Still, Dave Winfield is having a season worthy of his salary (.275, with 25 homers, 94 RBI and 19 game-winning RBI). Dave Righetti is becoming a great pitcher. The Magic Dragon, Graig Nettles, has defied his age (39) once more. Ken Griffey is hitting .330 and rookie Mattingly (.329) has one sweet stroke. Gossage (2.09 ERA) is still the Goose Egg man and Don Baylor (.290) has survived the I Just Became a Yankee Blues.

Nonetheless, Yankee Stadium remains a house of great discontent.

One week from today, the Yankees must start a rugged two-week West Coast road trip, followed by consecutive series against Milwaukee, Baltimore and Milwaukee.

If, four weeks from now, the Yankees are still in the race, then such folks as Anderson and Harrah will seem wise. With all that talent, how could they lose?

If not, Yankee Stadium will be hip-deep in alibis and bodies.

Next: the Baltimore Orioles