Its not a requirement its usually up to the team. It's hard to hide now with technology but imagine its the 90s and Barry Sanders breaks his foot in the off season. The lions try to hide the information and trade him for max value before the news come out. A team could trade for him and found out he was hurt after the trade. This is extreme scenerio and unlikely but alot of teams add pending a physical to prevent these type of trades from happening. But a team could also trade with knowledge of the injury in which they probably trade at a discount hoping to get huge gains on their investment.