According to Adam Schefter on ESPN, Stafford has a second degree shoulder separation. Shortly after, he put the following note out on Twitter:
QB Matthew Stafford has second-degree separated shoulder; planning to visit Dr. James Andrews as early as this week. Out this week at least.
The ticker on ESPN was immediately updated with confirmation of the Lions' reports that it's not believed Stafford will need surgery, but also added a potential timeline of 4-8 weeks for his recovery.
According to Dave Birkett, Stafford denied knowing exactly what it is:
Stafford on Mitch Albom show asked about 2nd degree separation said "I haven’t heard that" from doctors. Take it for what it's worth ...
Those might be Stafford's instructions from the team, or it might be the truth. We'll know more later this week, I'm sure.