Georgetown has looked less and less invincible the past three games. And even though they remained undefeated with a 61-56 victory over North Carolina A&T in the Copa Navidad tournament tonight, the top-ranked Hoyas suffered their first setback of the season when junior point guard Michael Jackson sustained a shoulder injury, the severity of which is undetermined.

Jackson, one of the nation's leading assist men with an average of eight per game, collided with an Aggie player midway through the first half in the game at Roberto Clemente Coliseum, and sat out the rest of the contest with his right shoulder heavily wrapped.

Coach John Thompson said Jackson's shoulder "popped out of place," and added that further examination would be necessary before a decision could be made on whether Jackson would miss any playing time for the Hoyas, whose next game will be Wednesday at Capital Centre against Seton Hall in the Big East opener.

In the tournament's second game tonight, Seton Hall defeated Tennessee State, 97-88.

In Jackson's absence, 6-foot-7 sophomore Reggie Williams played the point much of the night, and led Georgetown in scoring with 21 points.

"We certainly missed Michael; there's no question about it," Thompson said. "But Reggie did a great job. He's been preparing for this from Day One. He might be the second-best point guard we have."

With the Aggies often packing all five players in the lane in a 1-2-2 zone, they were able to hold down Georgetown's inside game. Patrick Ewing had 11 points, but got off only five shots. And Billy Martin, who tonight passed the 1,000-point mark in career scoring, had 10 points on only six shots.

That made Georgetown's perimeter play crucial. And it was junior guard Horace Broadnax who hit a jumper from the top of the key that ended a six-point run by A&T and increased the Hoyas' lead to 48-44 with about eight minutes left.

Ewing's steal and pass to David Wingate for a basket put the Hoyas ahead, 50-44. A&T's Jimmy Brown, son of the great fullback, challenged Ewing inside and came away with a layup for 50-46. And a Georgetown turnover set up an important possession for the Aggies (4-4).

The scoreboard was not working, and the 45-second shot clock A&T was using also had problems. "I thought there were about 14 seconds left on the shot clock," A&T Coach Don Corbett said later.

Instead, the shot clock expired. Broadnax then hit a jumper for 52-46 and, after A&T's Claude Williams missed a free throw, Reggie Williams sank a jumper that provided the Hoyas with an eight-point lead.

But Georgetown, which had defeated its first eight opponents by 20 or more points, couldn't pull away this time.

"Coming off the court, Corbett said, 'Thanks for holding the score down,' Thompson recounted. "I told him, 'I was trying to beat the hell out of you.' Corbett just did a hell of a job."

The Hoyas, on the other hand, couldn't, or at least didn't, largely because the Aggies' guards, Brown and Eric Boyd, played superbly. Each scored 18 points and combined for eight assists. Both hit long jumpers and both drove inside, showing no fear of Ewing.

"A lot of the problems teams have when going against Georgetown is that they play passively," Brown said. "We don't just believe anybody is better than us. They have to prove it."

Thompson said he went into the game cautious about playing a man-to-man defense because the Aggies were quick, "and they interchange a lot of positions," he said.

Martin pointed out that the A&T big men, as such, would try to take him and Ewing away from the basket if the Hoyas played man to man. Thompson even told Ewing and Martin not to guard their men away from the basket.

But after forward Claude Williams (10 points) hit three long jumpers, Martin looked back over to the bench, and Thompson said something to the effect of, "You better go get him."

Thompson, who was upset with his team in its 77-64 victory over Tennessee State here Friday night, said he felt, "a lot better about tonight than last night."