Will damaging his knee ligaments affect his pace at all? Surely it must feel a little funny in there now and it will be hard for him to keep up 90 minutes of pretending he's a Zonda R...
That's a good question.
Technically there is no reason why his knee should not feel any different to the way it did before the surgery.
HOWEVER, equally, ligaments control the whole structure of joints. If their size, plane of positioning, elasticity or possible alteration to the surrounding soft tissue are altered he most definitely will notice a difference in how the knee feels.
That MIGHT affect him physically or even psychologically. It all depends on the severity of the original injury and the skill of the surgeon.
There are no guarentees with ligaments.
I once was lying in a bed next to a guy who had snapped his anterior cruciate on a skiing holiday in Spain (this was in a hospital, I might add

). He had had an operation in a local hospital in Spain to re-attach the ligament and the knee had settled down. After recovery he found that although he had no primary pain he was unable to walk without a limp. An Orthopod in England scanned the knee and had told him that the ligament had been re-attached poorly and that even with further corrective surgery it was unlikley he would ever be able to walk normally again.
Ligaments are very serious things, far more serious than a bone break.
In answer to your question, there is a very real chance Stewart COULD never get back to the condition he was in before his injury.