Argentinian second row Ignacio Fernandez Lobbe, who was brought in from French club Castres, injured his knee playing for the Pumas against New Zealand in June and will be out of action for four months.
He will join the club when he is fit again after Christmas, but in the meantime Phillipe St Andre will look to plug the gap.
He said: "Ignacio had surgery on his knee a week ago and he will not be able to play for the next four or five months.
"It is a blow for us as we are trying to build a side capable of beating the best in the league, but it is one we will overcome.
"I will find a replacement before the start of the season."