Stafford was replaced by Shaun Hill for the last few drives of the Lions' 44-41 overtime loss to the Titans last Sunday. It wasn't immediately apparent what kind of injury Stafford sustained, but an MRI revealed that he had a hamstring injury. He's expected to play against the Vikings as long as the injury isn't reaggravated in practice.
Stafford is off to a bit of a disappointing start to this year, as he has more interceptions (4) than touchdowns (3). However, he's completing nearly 70% of his passes. A prolonged hamstring issue for Stafford would be a huge blow to the Lions' playoff chances, which are already in trouble after a 1-2 start.