For most of his 27 years, John Lucas has been on top of the world: an all-America at Maryland, the first man picked in the National Basketball Association draft five years ago, a highly skilled tennis player who won an Atlantic Coast Conference championship. He was popular with fans and the press, an extrovert who exuded confidence.

Until now.

Today, John Lucas is a troubled man whose basketball future is in doubt. There are people around the NBA who speculate that Lucas' problems are partly related to drugs or alcohol. He vehemently denies it. "It's none of that," he says.

Still, no one in the Golden State Warrior organization wants to hear Lucas' excuses anymore. But no one, except John Lucas, knows what is wrong. They only know the model citizen, the Mr. Nice Guy, has changed. And, his friends say, no one can figure out why.

Once the star of the Warriors, Lucas hasn't shown up for four games this season and has missed more than a dozen practices. According to the Warriors, his excuses are lame and the club is in the process of deciding whether to give up on him.

"The last time I talked to him, he didn't even sound like himself," said Warrior Coach Al Attles. "Everything he's done lately is so out of character.He's done a lot of erratic things and when you talk to him about it, it's apparently for no reason."

"I have some problems that I just can't discuss," Lucas said yesterday. "But I don't miss games without a good reason. A big part of what's going on is that I'm probably just tired of playing basketball and tennis and everything. I've been doing it since I was 7 and it could just be getting to me now. I just don't know if I want to play basketball anymore.

"I don't think the team has been completely fair with me, either."

Attles disagrees.

Both Attles and the Warriors' owner, Franklin Mieuli, have a reputation for being fair and understanding. But their patience has run out.

"I've done everything I could have done to try and understand and help John," Attles said. "But things have started to bother his teammates now, and so something has to be done. I want to find out what's wrong and help, but John is making that difficult."

Last Saturday, Lucas called Attles and told him he knew he had been causing problems. He pledged not to give Attles or the team any more trouble. Then, he failed to show up for the game that night against the New York Knicks and he failed to show up for practice the next day.

Lucas called the Warrior office Monday, but by then Attles had had enough. He told Lucas he didn't want him to go on the three-game trip to Chicago, Indiana and Detroit. He told him to take that time off, as well as the all-star break, and reevaluate his life.

At the same time, the Warriors are reevaluating Lucas.

Attles says they don't want to give up on him, even if he hasn't been playing very well when he does show up. On the other hand, no other teams are burning up the phone lines trying to take him off the Warriors' hands. Lucas never was a good defensive player or shooter. This year, every one of his offensive statistics is far below his career averages.

Lucas signed a unique, five-year contract with the Houston Rockets when they made him the first player picked in the 1976 draft. There is a clause in the contract that prohibits his team from suspending him without pay. That is one reason the Warriors have been so lenient.

The contract also contained a one year option. The Warriors decided Jan. 15 not to exercise that option, which means Lucas will be a free agent at the end of this year.

Attles has talked to Lucas' parents and his sister. He says Jesse Jackson, a friend of both Attles and Lucas, has talked to Lucas to see if he could help. Lucas, Attles said, simply has told everyone who inquires that everything is fine.

"But obviously it isn't," Attles said. "He asked me once why people were so hard on him and saying bad things about him. I told him it was because he was doing things that are so out of character, and as long as he continues to do that, what does he expect?

"He's got serious problems and I just hope they're basketball related. He's got to get himself together before anyone else can help him, but people are starting to take a different look at him now and speculate that all sorts of things are wrong with him. I hear the thing about drugs, too."

Lucas didn't miss a game his first two seasons in the league, with Houston. When Rick Barry, formerly a Warrior, signed with the Rockets as a free agent, Lucas became part of the compensation the Warriors received. a

Lucas was just as big a hit at Golden State as he had been in Houston. The coach loved him, his teammates loved him and the fans loved him. He didn't miss a game his first year with the Warriors and, last season, missed only two, both for legitimate reasons.

And then, as the Warriors changed this season, so did Lucas.

According to sources on the team, Lucas had a hard time coping with his new role. He had always been the star, the man at center stage, orchestrating the show. Even if he wasn't playing on a winning team, it was his show and he loved the limelight.

"For two years Luke got all the attention, and now all of a sudden Bernard (King) and All-World (Lloyd Free) are getting it," Attles said. "Outwardly, John may give the impression that he's cool and confident, but I think he's a little paranoid. Sure his game has to change because of the new people, but he could still play if he wanted to."

"I can't be worried about how John feels," Free said, "but if he's upset at his role or something, you still don't do things the way he's doing them."

"We've done everything we can legally do to him," Attles said. "We've fined him, sent him home and everything else. We have normal procedures you have to go through if you miss a game or a practice and he doesn't go through them. What can we do? I've stayed in contact with him and talked to people about him. No one is picking on him, even though he says he feels like they are. People just want to know what's wrong with him.

"If there's any way anyone can help him, he has to let them."