Waller blow for Saints
The 27-year-old was withdrawn in the 78th minute and has since undergone surgery.
He will require eight to ten weeks rehabilitation.
Director of Rugby Jim Mallinder said: "We're obviously very disappointed to lose Alex.
"His run of consecutive starts in the Aviva Premiership – stretching all the way back to May 2011 – has been remarkable.
"But he's a resilient player and will be working hard to come back stronger for an important period in the new year."