Onuaku landed awkwardly after trying to block Greg Monroe's shot down low. Whistled for a foul on the play, he grabbed his knee immediately and was tended to by medical staff and coach Jim Boeheim before being helped off the floor. The senior did not return and is expected to get an MRI exam.
Onuaku, who has struggled with knee pain throughout his career, had surgery last spring.
"He has a strain. We don't know for sure if it's any more than that. We'll find out," Boeheim said.
Georgetown's Austin Freeman added 18 points in his third game since being diagnosed with diabetes. Jason Clark had 17 for the eighth-seeded Hoyas (22-9), who will play Marquette in the first semifinal tonight.
Monroe had 15 points, 10 rebounds and a team-high seven assists, skillfully dominating in the paint against Syracuse's big and rugged front line.
"It's definitely a confidence boost," Wright said. "I don't think this team lacks confidence. I just think we're ready to play."
Playing in front of a lively crowd, Georgetown rebounded from two losses to Syracuse earlier this season and took a 7-6 edge in Big East Tournament games between the longtime rivals, the most common matchup in the 31-year history of the event.
Five times they've played for the title, with the Hoyas winning the first four before Syracuse broke through in 1992.
Syracuse, which won the regular-season conference championship, has dropped consecutive games for the first time all season.
Conference player of the year Wes Johnson led Syracuse with 24 points and seven rebounds in his Big East Tournament debut. Neumann-Goretti product Scoop Jardine added 19 points and Kris Joseph 18, both off the bench. Andy Rautins, who had 14 points and 10 assists, keyed the spurt that gave the Orange a nine-point lead with 13:43 left, then provided little down the stretch.
In the other quarterfinals:
