Lawes suffered the problem due to a fall in training last week and was forced to withdraw from the squad ahead of the clash with Wales.
On Wednesday afternoon, it was announced that his Saints team-mate, David Ribbans, has been called up as a replacement by England.
And now Saints have confirmed that Lawes will have to undergo surgery, ruling him out for approximately 12 weeks.
The Saints statement read: "Northampton Saints can confirm that, following consultation between the club’s medical staff and a specialist, Courtney Lawes will undergo surgery on the pectoral injury he sustained in England training last week.
"The 32-year-old lock will require a rehabilitation period of approximately 12 weeks."
The news is a big blow for Saints, but should Lawes return inside a three-month time-frame, he could still play a part in this Gallagher Premiership season.
Saints do not conclude their regular-season campaign until the weekend of June 12.