The way I see it is;
They have space. They are short of a prop, for what will be an intense end to the season, whatever happens. They don't really have much to loose, as if he's plays crap, they are no worse off than they are now. In all reality could they have signed anyone else who isn't cup tied at this stage of the season?
Having said that not sure I would be creaming myself if we had bought him.