Well, if Morgoth's feet were chopped off, then how did he managed to escape from the Void? how did he slipped past the guards? If he started a final battle, than he had to have some sort of power left in him, hadn't he?
Btw, I don't think Tulkas would've needed any assistance: he took care of Melkor alone before!
Evil may yet be good to have been ... and yet remain evil. (Silmarillion)