It is a big blow for the reigning GUINNESS PRO12 champions with MacArthur having shone for both club and country in recent times.
But after realising that MacArthur will play no more rugby in 2015 Townsend knows his full recovery is now the most important factor.
"Pat has featured regularly for us this season and has been in good form, while also stepping up as a leader, so it's disappointing to lose him for the next few weeks," Townsend told the club's official website.
"He is due to be discharged home today [Wednesday] and we look forward to him re-joining the squad when he has fully recovered."
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