With Matt Hasselbeck scheduled for an MRI tomorrow morning, this might be a good time to take a look at various grades of knee injuries. The Seahawks announced Hasseleck's injury as a sprain. They said his return was doubtful. They let him walk to the locker room. Those things tell us this was not a catastrophic injury. He was hit on the outside of the knee, which tends to put strain on the medial-collateral ligament (MCL). According to information from Harvard Medical School (see above link), "How long a knee sprain lasts depends on the type of knee sprain, the severity of your injury, your rehabilitation program and the types of sports you play. In general, milder Grade I and Grade II MCL or LCL sprains heal within 2 to 4 weeks."
One thing I've noticed about Hasselbeck in recent years is that he has really beefed up his legs. He was lanky when the Seahawks acquired him in 2001, but not now. The added strength might have saved him from a more serious injury.