the medical tests would have filtered anything out.
it's not like Karl-Heinz Rummenigge pointed a gun to Ferguson's head and went "BUY HARGREAVES NOW EVEN THOUGH HE'S INJURED!"
assuming that the rumour is true, it'd be Manchester United's fault for deciding to buy a player because of the injury that he supposedly had hidden.
remember, Ruud van Nistelrooy's transfer was delayed by one season because of his knee ligament issue. so I don't think Man United would have been THAT stupid to buy Owen Hargreaves under an injury cloud.