Derrick Jackson, Georgetown's star basketball guard, remained hospitalized last night and Coach John Thompson moved to overhaul the Hoyas' starting lineup for tomorrow night's first-round National Invitation Tournment game at Virginia.

Jackson, suffering from a bleeding ulcer, had been expected to be released from Sibley Hospital yesterday. Thompson said Jackson's extended hospitalization was a precautionary measure. He is expected to join his teammates in Charlottesville tomorrow afternoon. His availability against Virginia is expected to be limited to token appearance.

Following Jackson's hospitalization shortly before the Hoyas' ECAC play-off game loss to Virginia Commonwealth last Wednesday, Thompson replaced Jackson with 5-foot-8 senior Mike Riley, a defensive specialist who serves the Hoyas better as a substitute.Thompson said yesterday that Riley would not start against Virginia.

Thompson worked on two alternatives during yesterday's practice:

Forward Steve Martin, a point guard in high school and the third guard his first two seasons at Georgetown, moved to John Duren's point-guard slot, with Duren moving into Jackson's position and Al Dutch replacing Martin at forward. This alignment seems to be the one Thompson prefers.

Reserve guard Craig Esherick, a senior best known for his 35-foot shot that enabled the Hoyas to eventually beat George Washington earlier this season, also played in Jackson's position, with no other changes in the starting five.

Jackson appeared at practice briefly yesterday for a picture-taking session but returned to Sibley.

By tomorrow night, Jackson will not have practiced or worked out for a week. Even if he is physically able to play, as he says he is, he probably would have little stamina.

Thompson said he would be willing to switch two starters to other positions in order to keep Riley where both he and his defensive specialist prefer when the game starts - on the bench.

"You don't have another gear when he starts," the coach said.

Georgetown's game is predicted on quickness. The Hoyas have used speed to rally in at least three games this season, once from 22 points down with 12 1/2 minutes to play. When Riley enters a game, it gives notice to teammates and opposition that something is going to happen.

"To say Riley is quick isn't paying him credit for half of his ability," said Thompson. "He's quick with his intelligence, too. I watch him and he reminds me of a big cat in the jungle setting up his prey.

"He is extremely valuable. I'm trying to recruit someone like him. They're hard to find. They have the quickness, but isn't harnessed. Rarely do you find a player who treats defense as an art of the game. Most players find it an inconvenience."

Riley, who served three years asa Navy missile technician after high school, said he realized long ago that it would take his defense to make him a college basketball player. "In the recreation leagues, I like to score like everyone else," he said. "But here I know what I have to do to play."

Thompson first noticed Riley when he was a Cardozo sernior and Thompson was still coaching at St. Anthony's High School.

"I took my team to watch Cardozo play at De Matha," Thompson recalled yesterday. "He was playing against Ray Hite (who later played at North Carolina). "He was the first high-school guard I saw who dominated a game defensively. I turned to my players and said, 'That little boy will never play in college because other coaches will say he's too small.'

"Then I got here and I remembered him. Somebody said he was in the service."

Riley was on the USS Holland handling Polaris missiles and nuclear weapons. When the ship returned to port, there was a message to call John Thompson. When Riley was discharged he signed with the Hoyas.