MILLBRAE, CALIF. -- Baseball people talk of Gregg Jefferies so positively that when you first see him he doesn't impress you.

Is this unassuming, 5-foot-10, 170-pound infielder with the dark hair and squinty eyes the same Gregg Jefferies who played so phenomenally in three minor league seasons?

Is this harmless-looking 20-year-old the player who has averaged .354 in the minors and who has won two successive player-of-the-year awards?

It is not until you see him handle a bat, albeit casually and effortlessly, that you begin to believe. And once you do, the phrase "limitless potential" comes to mind, as it has to so many baseball people who have seen him play.

His boyish appearance and calm manner are part of what make him so compelling. Here's a young man from the outskirts of San Francisco who can hit a ball all over the park with the same sweet stroke from either side of the plate. He can steal bases, bunt and has surprising power.

And all that comes in a neat, unpretentious and mature package. Add in the father who travels on the road with him during the summer to provide a stern hand and the quiet mother with the calming influence at home.

"The whole package is quite intriguing," said Steve Schryver, the Mets' director of farm clubs. "He's not a natural. I've seen many players with better physical skills. But what sets him apart is that he has tremendous dedication and motivation. He's almost in the category in the strict definition of 'unique.' "

What really sets him apart are his achievements. He was the Mets' first-round pick in the 1985 draft; he played Class A ball in Kingsport and Columbia, batted .312 in 67 games and was the league's rookie of the year and player of the year.

In 1986, he batted .339 with Columbia, .354 with Lynchburg and .421 in five games with Jackson of the Texas League in Double-A. Again, he was player of the year. Last season, he averaged .367 in 134 games with Jackson, including 48 doubles, five triples, 20 home runs and 101 RBI. He stole 26 bases. He failed to beat out only one bunt. At the end of the season, one in which Baseball America magazine named him player of the year, he was called up to the Mets.

"When I was told I got called up, I thought it was to Triple-A," he said. "When I heard it was the Mets, I was shocked. My teammates said I was as white as a ghost."

He got three hits in six at-bats with the Mets, including a double off the center field wall at Shea Stadium, and with a bad leg, no less. He was impressive enough that Lenny Dykstra and Gary Carter sent him Christmas cards.

"Gregg's the best prospect I've ever seen," said Tucker Ashford, Jefferies' manager at Jackson. "He's so mature for his age. But then again, he's been so successful that the most immature person in the world with Gregg's achievements could come off looking mature, because it's tough to be immature when you're so successful."

Winter is a time for rest, relaxation and reflection; a time to get away from the game, to enjoy the rewards of being the finest prospect in major league baseball. But Jefferies has spent the winter raising his already lofty standards.

In what even he calls a "crazy" regimen, he has spent six days a week going through unconventional and strenuous workouts that he and his father Rich devised and believe contribute to his prowess.

Among them, he swings a sawed-off bat under water 25 times each as a right-hander (his natural side) and as a left-hander. "If you ever walked in a pool," explained Rich Jefferies, a junior high baseball coach and a scout for the Chicago Cubs, "you'll notice the resistance the water gives. So, we figured if you swung a bat under water there'd be the same resistance and it would help in developing bat speed."

His son lifts weights, runs, punches a speed bag, swings a leaded bat into a heavy bag and throws a football 70 times, among other things. The workouts last eight hours.

"Sometimes I feel like I'm a boxer," he said. "But for me, this is work, like a 9-to-5. I'm not a guy who'll hit the ball 500 feet. I need an advantage."

Said Ashford: "I've never heard of someone doing the things he does. But I think the theory is great. He's as strong at the end of the season as he is at the beginning."

But Jefferies doesn't do as well defensively as he does offensively; primarily, says the Mets' Schryver, because he's judged by his offense. Jefferies was drafted as a shortstop and, after having some troubles there, has played some third base.

"His defensive skills are normal," Schryver said. "His offensive talents vaulted him into the limelight, and in the cross-examination his defense is unfairly criticized. His defense is normal, but his offensive skills are not normal.

"He's no Ozzie Smith. He doesn't have a rocket arm, and he still gets his feet crossed up sometimes. That's normal. But Ozzie Smith can't hit with Gregg."

The feeling is that, years from now, young big league prospects will cover their bedroom walls with posters of Gregg Jefferies. The pressure to produce in New York will not engulf him, he said, because "they already have a good team. It won't be like with Darryl {Strawberry}. He was sort of in the same situation. When he came, they were fading, and he had to produce."

He's scheduled to start this spring with the Mets' Triple-A team in Tidewater, Va. When he will get a real shot with the Mets, no one is ready to say. But it is not far off. For his part, he said: "I'm not ready and the Mets know it. I've had a few good years. There's still so much to work on. I'm just soaking up information in the minors, especially about learning pitchers and their patterns."