This has been a season of surprises for the basketball team at De Paul University, the Vincentian school beside the tracks of Chicago's northside elevated train. Not all have been pleasant.

Early this month, starting junior forward Teddy Grubbs, a 1981 honorable mention member of Street and Smith's all-America team, temporarily left school because of what Coach Ray Meyer termed acute psychological problems.

Senior Skip Dillard, the cocaptain and shooting guard, remains, in his own words, "psyched out" after missing the first shot of a one and one against St. Joseph's, with the game on the line, in De Paul's first game in last year's NCAA tournament. Mark Aguirre, their petulant superstar, turned pro after his junior season. And yet the team is ranked third in the nation, with a 24-1 record.

The Blue Demons are winning because of defense--a game element that has not been one of the team's strong points in past seasons. The Blue Demons, a perennial run-and-gun team, have held opponents to an average of 64 points per game this season, their best defensive yield in 30 years.

An aggressive man-to-man coverage has pressured opponents into shooting 42.5 percent (De Paul ranks eighth in this category), and into committing almost 20 turnovers per game. Led by quick-handed guards Dillard and freshman Kenny Patterson, the team has averaged better than 10 steals a game. "Defense is the real key," said Meyer, in his 40th season at De Paul. "How else could we win 24, shooting just 48 percent?"

This newfound appreciation for defense--like many other attributes of this De Paul team--is a response to the void left by Aguirre. "Sure, we made this commitment when we lost Mark," Meyer said. "We had to, because we weren't sure we could score. With Mark, we figured it wasn't a matter of defense, just outscoring everybody. Without him, we had to make defense our strong point."

It has been strong enough for the team to withstand Grubbs' abrupt Feb. 2 disappearance from the team. Grubbs entered De Paul with a reputation more glowing than that of his teammate, all-America forward Terry Cummings. In a substitute role in the fourth game of his freshman season, Grubbs scored 28 points and got five rebounds in 31 minutes in a nationally televised 99-94 win at UCLA.

After that, he struggled to match the expectations he created, and was unable to do so. By the time Meyer reluctantly benched him last December, he had virtually ceased to speak or change expression.

"I tried everything to reach him," Meyer said. "Talking to him. Belittling him. Encouraging him. Ignoring him. Chewing him out. I've never reached him.

"We felt that he was going to be a super ballplayer this year. Prior to this season, I thought he was going to be out of this world. He played so well here when I used to watch them play on their own before we started to practice this season. He'd be out there playing, and God, he looked great.

"But Teddy's had a problem . . .for quite awhile now. He's back in school now, attending classes. He was out for about a week, 10 days."

A De Paul official said that Grubbs, a 6-foot-8 junior, has been undergoing therapy for about a year. He is not expected to return to the team this year.

Dillard, the senior guard who once roomed with Grubbs, faces problems of his own. He has been in a prolonged shooting slump, from which he is just emerging. Through early February, he had shot just 44 percent from the floor, down from 52 percent the year before.

His free-throw shooting has been even more disappointing. Since missing that shot against St. Joseph's, the man nicknamed "Money" for his sure touch has been small change. In a Feb. 3 rematch against St. Joseph's, Dillard was again on the free-throw line with the game in balance. Again he missed--this time both ends of a two-shot foul. "I used to be a great free-throw shooter," he said. "Now I'm just trying to be a good one."

Left to carry the offense--and, for that matter, the entire team--has been cocaptain Cummings, a charismatic 6-10 junior. Cummings, an ordained fundamentalist minister, is the team's leader. "Terry wants the leadership role," said Meyer's chief assistant and heir apparent, son Joey. "That's his personality. He wants the team to do the kinds of things that he thinks it should do, and he's a strong enough person to go ahead and do that. He's a dominating force."

Since moving from center to his natural, power forward position, Cummings has become to this team what Aguirre was to last year's squad. He is averaging almost 23 points a game, 14th in the nation. He ranks 12th in rebounds, with almost 12 per game. From inside, he has been unstoppable. "When we're in trouble, we just look to Terry," said freshman center Walter Downing. "He's never failed to save us yet."

Cummings has been instrumental in the rapid development of starting point guard Patterson, and freshman sixth man Tyrone Corbin, a strong rebounding forward. He has worked hard to prepare them for the coming NCAA tournament, which De Paul has entered two years in a row as the No. 1-ranked team, only to exit each time in its first game.

"I don't believe in jinxes," Cummings told his teammates. "I don't care if we lost 10 years in a row in the first round. We're going to get past the first round this year. I know so."