You can't get anyone with worse current form than i dead guy so it makes the point perfectly clear.
McDonald was given a trial by word of mouth or previous form but he was signed after being watched. Unless there was no members of the management team watching him when he was having his trial.
The point is unless you are Souness then all players are signed after being watched by management/scouting team so therefore current form with whoever you play for is the key factor in signing a player. Saying someones current form is irrelevent because it's with another club is tosh.