Zylstra suffered a fractured kneecap in the Lions' loss to the Falcons on Sunday, Adam Schefter of ESPN reports.
Zylstra had seen his role grow with the Lions since T.J. Hockenson went on IR with a thumb injury. However, the knee injury almost certainly puts a crimp in those plans. Although not officially ruled out for Week 17 against the Seahawks, it's unlikely Zylstra will be able to play.