Coach Lawrence Frank called the injury a sprain, but said Yi will undergo an MRI on Tuesday to determine its severity.
Yi was near the basket when Bobcats forward Gerald Wallace fell and rolled into his leg midway through the third quarter. Yi crumpled to the floor and was attended to for a couple of minutes. He eventually got up and put no pressure on his right leg as he left the floor, and the team announced he would not return.
"I fell on my knee and kind of twisted it a little bit," said Yi, who had four points and six rebounds before the injury.
It was yet another blow for the Nets (0-4), who played their second straight game without point guard Devin Harris (groin). Keyon Dooling (hip), Tony Battie (knee) and Jarvis Hayes (hamstring) are also sidelined.
Chris Douglas-Roberts rolled his ankle late in the fourth quarter, but stayed in the game.