OK, now we have our answers. Wikipedia reference-linkFutsal is a very demanding sport on the legs. As the ball doesn't bounce as much as a normal football the players technique is called into action here. It would be common to see players with medial ligament and PF joint line tenderness playing this game. He may have some medial laxity making it worse? You could strap him prior to playing one practice session to see if that helps. Otherwise you'll need to look at the patient in the game to see what they do. Consider options in the footwear also as it is played on a hard surface as well.