Lovie Smith just had his Monday press conference. Per multiple twitters:

Sean Jensen:

Lovie Smith confirms that it was an MCL injury. #Bears QB Jay Cutler could have been more seriously hurt.

Jeff Dickerson:

Cutler likely could have played in two weeks with sprained MCL, but not in the second half after suffering the injury. JD

Information on injury type:

From The Stretching Institute:

MCL tear, or MCL sprain, is another common knee injury, which typically affects athletes involved in contact sports, such as football, gridiron and soccer. MCL tear and MCL sprain are commonly referred to as a Medial Collateral Ligament Injury.

So a sprain and a tear are considered the same thing.

What causes Medial Collateral Ligament Tear?
By far the most common cause of medial collateral ligament sprain is a blow or sudden impact to the outside (lateral) of the knee joint. This causes the outside of the knee to collapse inward toward the midline of the body and the inside of the knee (where the medial collateral ligament is located) to widen and open up. This opening up stretches the ligament, which results in the injury.

PERHAPS THE MOST CRUCIAL:

Rest & Immobilization
Once a MCL injury is diagnosed it is important that the affected area be rested immediately. Any further movement or stress will only aggravate the condition and prolong recovery. It is also important to keep the injured area as still as possible.

He should have just played through it, right?