Most typically, a direct blow to the outside of the knee would include damage to the medial collateral ligament, the structure that runs along the inside of the joint -- connecting the femur (thigh bone) to the shin bone (tibia) -- and keeps the knee from rotating sideways. Without damage to other structures of the knee, an MCL sprain alone generally does not need surgery, and his recovery would depend on the grade of the sprain.
By definition, sprains involve some degree of tearing of the ligaments: a Grade 1 sprain, for example, involves only a small amount of ligament fiber damage, whereas a Grade 3 sprain translates to a complete tear. The expected timetable for recovery for MCL sprains generally are 1-2 weeks for a Grade 1 sprain, 3-6 weeks for a Grade 2 sprain and up to 2-3 months for a Grade 3 sprain.
When Roethlisberger does return -- early indications were that the Steelers were anticipating a 4-6 week recovery period consistent with a Grade 2 sprain -- he will almost certainly require a brace on the left knee, which may affect his mobility.
The x-factor in his recovery time is the bone bruise, which could add to the timetable if it doesn't heal as quickly as the ligament. For now, though, the team is undoubtedly relieved that it is just an MCL sprain rather than other alternatives.