It is the day after the Detroit Pistons have beaten the defending champion Boston Celtics, 125-117, in Game 3 of the NBA's Eastern Conference semifinals.

This should be a relatively happy time for Isiah Lord Thomas. The Pistons' all-star guard had 26 points and 16 assists in the Thursday game. Yet, as he meets a group of reporters in front of his locker after an early afternoon practice, the only trace of emotion Thomas shows is the scowl on his face.

The Pistons' public relations director takes Thomas aside, asking him to loosen up a bit. The request is denied. Only after the media horde has departed does Thomas relax. "The press has a job to do and I understand that," he said. "But it seems like if you're not saying anything controversial, not getting into a words war, they don't want to hear it. They walk away and act like I'm not helping them. I guess it's their problem."

As recently as last season, Thomas would have fretted for hours over the incident, assuming that the fault was his. Then again, it probably never would have happened. Often described as "the cherubic (or impish) Isiah Thomas," the first-round draft choice from Indiana in 1983, who turned 24 on Tuesday, is one of the most ebullient players in the league, his smile always ready for the next set of television cameras.

His actions Friday are part of a transformation taking place inside Thomas. "I can't say in what ways, but I'm pretty sure it's there," he said. "I'm not one to sit back and analyze myself, but I know that it's happening. I guess I'm developing as a person."

Perhaps more than anything else, Thomas has become his own person. He's no longer a "little angel," no longer "Pocket Magic," a somewhat belittling reference to 6-foot-9 Earvin Johnson of the Lakers, another all-star who also happens to be Thomas' best friend.

These days, he's simply Isiah Thomas, basketball player, which, looking at the numbers, isn't a bad thing to be. An all-star in each of his NBA seasons, Thomas averaged 21 points a game and set a league record for assists (1,123) in the 1984-85 regular season. In the playoffs, the scoring has increased to 23.8 points and the assists continue at 13 per game along with 51 percent field goal shooting.

That Thomas' playoff statistics are better, that the Pistons are in the second round of the playoffs for the first time since 1977, and that there seems to a grimmer countenance surrounding the guard aren't random coincidences.

"I think up to now he's been searching for things, where he belonged -- in groups, this team or in the league. Now it's just established that he's a great player, that's his status in the game," said Detroit center Bill Laimbeer. "He's the authoritative figure on our team, what he says is the final word on how things will go for us."

Consider that these words are being spoken by a 6-11, 260-pound man. Laimbeer is also an all-star, playing a position that is the focal point on most NBA teams. That it is not that way with Detroit gave Coach Chuck Daly pause for concern when he was hired two years ago but not today.

"It's a bit strange being so dependent upon a 6-foot-1 guard, but you have to go with the personnel you have and obviously Isiah is an outstanding player," Daly said. "There's a big load placed upon you with any success but in the time I've been here, Isiah's only become more mature, his concentration level is more consistently higher."

Even as an all-star, the knock on Thomas' game throughout his brief career has been that there was a tendency for him to run amok, or, as Daly put it, "to go off on sorties."

That, too, has changed, according to Daly. "A lot of coaches won't give a guy free rein. When you do you have to live with shots being taken with only five seconds gone on the shot clock," Daly said. "There is a play that I can go to to control the situation, to make Isiah become purely a passer, but I don't think I've had to call it more than twice a game this year. What he does, the decisions he makes are the best thing for our club."

According to Laimbeer, the only time he's seen Thomas play out of control this season was during the third-to-last regular season game. "People had been really after him about when he was going to break the (assists) record. He needed 21 and he was playing like he wanted to get them all that night," said Laimbeer. "He finished the game with something like 10 turnovers because he was trying plays that he wouldn't normally make. I don't think that night was really his fault."

Sometimes the decisions made by Thomas involve immediate, hands-on action. In a game dominated by behemoths, perhaps it is Thomas' diminutive stature that creates such a big stir when those moments occur. The ultimate example came in the fifth game of last season's first-round playoff series between the Pistons and New York Knicks.

Detroit lost in overtime, 127-123, but it wouldn't have been nearly that close without Thomas. In the final quarter, Thomas scored 21 of his 35 points. Of those 21, fifteen of them came in the last 1:57 of regulation. "There are times," Daly said, "when he feels he can go one-on-five and that no one will stop him."

It's hard to fathom, let alone describe, such incandescent moments. Thomas is past the point where he even bothers to try. "The game isn't that complicated to me. All I'm doin' is playin' ball," he said. "People ask me to break things down, to describe the science of passing. Man, by the time you'd try to stop and think about all that, the play's over."

Some teammates, however, say that Thomas' greatest contributions come away from the basketball. In fact, Thomas may be proudest of his leadership qualities. On the Celtics' first possession after Boston's Robert Parish leveled Laimbeer with an elbow during Thursday's game, a shot by Larry Bird was hindered by a Thomas forearm to the head.

Perhaps he was trying to block the shot, perhaps not. "If one of my guys is fighting, in trouble or needs help, then I've gotta be there. If I'm with you, I'm with you all the way," Thomas said. "I'm not gonna sit around and watch. Even if I get beat up, at least I can say, 'You got a black eye, I got a black eye, but we were there.' That's a lot different than, 'Man, you really got messed up.' "

Of course, there are more subtle forms of leadership, and Thomas is not hesitant to express them as well. "I think that's really my job, to make everyone on the team happy, to keep them away from problems, off the court and on. Chuck may yell at a guy during practice and the player will pout. I'll go to the guy and say, 'He's telling you right, just don't take it personally because he's yelling.' "

In a sense, Thomas is so readily able to impart such advice now only because he's taken it to heart himself. "If someone thought of me as 'Isiah the jerk' or 'Isiah the ass,' I guess I'd try to correct it because I don't want to give off that image, but I don't let things bother me as much now.

"If they still want to think of me as a little Magic, that's okay, too, I really don't care. It doesn't bother me, I'm very comfortable with myself. I respect myself and I feel secure."