Matthews, 22, suffered the injury at a workout with the Boston Celtics.
Matthews will have knee surgery in his hometown of Chicago in the near future, Pensack said. A timetable for his return will be established after the surgery.
After a strong showing at the NBA combine in Chicago, Matthews was garnering interest in the second round as one of the top defenders in the 2019 NBA draft class.
He is ranked No. 60 among the top 100 prospects available for this month's draft, according to ESPN.
"Injuries are tough, but Charles is an extremely hard worker and will be back stronger than ever," Pensack told ESPN. "He had a series of outstanding workouts lately and has helped himself. Charles will continue that momentum when he returns to action. As of now, we have interest in the second round as Charles is OK signing a two-way contract like Edmond Sumner did with the Indiana Pacers in 2017."