Shurmur made a stupid decision to call a wildcat play on a drive that was moving well. That flipped the momentum in Detroit's and put the team in an awful situation. Cook then fumbles and gets hurt on the next series, but he wouldn't have been in that position if they had not killed the previous drive with the wildcat.
It sounds like they are concerned that Cook tore his ACL, which is consistent with him being able to walk after his injury. We really need Bradford back, because things get a lot harder on offense without the threat of a running game. We have better depth with Murray and to a degree McKinnon, but it is a huge dropoff from Cook.