Hey there.
An operation such as that can lead to pains in the calf quite often as excess fluid from within the knee joint passes down the leg....and as the other poster said for all manner of other reasons related to bruising and so on.
However if your husband has a history of dvt or blood clots you should monitor the situation VERY carefully.
Main signs to lookfor are:
Pain in the leg
Tenderness in the calf (this is one of the most important signs)
Leg tenderness
Swelling of the leg
Increased warmth of the leg
Redness in the leg
Bluish skin discoloration
Discomfort when the foot is pulled upward
However dont forget that some of these symptoms can be from the op itself. Like tenderness and pain.
If however your husband is suffering from a suspicious amount of symptoms from this list then dont hesitate and get to your local A & E (or ER depending where you live) immediately. DVT is not something to be trifled with

Good luck and keep us posted