He left his team's 100-97 victory over the Boston Celtics on Saturday at TD Garden after injuring the knee late in the third quarter. He did not return, due to what the team called a sprained right knee.
At the time the injury occurred, it looked much more serious than that. As Jack planted his right leg on a fast break after receiving a pass from teammate Wayne Ellington, his knee buckled, which caused him to go down to the floor. He then had to be helped off the court.
"I had never felt pain like that before, so I knew it was something," Jack told reporters in Boston. "I'm going to try to stay positive and try not to speculate. I'm still optimistic, still hoping for the best, and tomorrow we'll find out what the future may hold."
Jack told the medical staff he did not want crutches, "but they said it's better to have them than not, so I'm about to go back and get fitted."
He told reporters he had never suffered a knee injury before. While he didn't know the extent of the injury, he posted on Instagram that it was the "worst pain I've ever felt in my life."
Jack left Saturday's game with two points and nine assists in 23 minutes. The 32-year-old veteran came in averaging 13.2 points, 7.3 assists and 4.4 rebounds while shooting 39.7 percent from the field.
The other point guards on Brooklyn's depth chart are reserves Shane Larkin and Donald Sloan. With Jack out, Larkin went 0-for-3 from the field with four turnovers and three personal fouls in the fourth quarter, as the Nets (10-23) nearly blew an eight-point lead in the final minute.
Earlier in the game, Jack exchanged words with Boston's Jae Crowder after Jared Sullinger committed a hard foul on Brooklyn's Brook Lopez underneath the basket. Following an official review, Sullinger was given a flagrant foul 1, and Jack and Crowder each received technicals.
"[I] wasn't trying to escalate the situation or anything like that," Jack said. "I just felt the play [Sullinger] made wasn't really a basketball one, and people can get hurt, especially when you do things up around the neck area with force. And [I] just let them know that we can't tolerate stuff like that, especially to our big men. So I was going to have [Brook's] back or anybody else's back in that situation."