Spence was in pursuit of Panthers QB Jimmy Clausen who was escaping the pocket to his right. Spence was hit by a running back as Clausen was moving past him, and Spence came down awkwardly on his leg.
It appeared to be at least a hyperextension, and judging by his reaction, it looks like it could be worse than that.
Trainers are on the field with him, along with several members of the team and head coach Mike Tomlin, as they are fitting his leg with an air cast, preparing to cart him off the field.
The Steelers lost rookie RG David DeCastro to a knee injury in the Steelers' Preseason Week 3 win over Buffalo. They lost FB/TE David Johnson to a torn ACL against Indianapolis in Week 2. OT Mike Adams suffered a minor knee injury in the preseason opener, but Max Starks and Casey Hampton tore ACLs in the Steelers playoff loss at Denver. Rashard Mendenhall tore his ACL against Cleveland in Week 17 of 2011.