Powerhouse centre Tuilagi, dropped by England for their opening two matches this Six Nations, was sent off for a forearm to the face of fellow England international Tommy Freeman in the 14th minute at Franklin's Gardens.
"He (Tuilagi) is gutted," said Sale boss Alex Sanderson, whose side remain on course for the play-offs despite this loss.
"He's apologised and said he owes us," he added of the 31-year-old Tuilagi, capped 50 times by England. "He fully owns what happened out there and that's the kind of man he is.
"I know he will pay us back but all of that is tough to take when you're still reeling from a game that we could have won and we probably should have won.
"This was a game where he wanted to show how good he still is to England and those guys who are watching."
Despite Tuilagi's transgression, Sale's physicality up front meant they were 24-7 up at half-time through tries from Arron Reed, Tom Curry, Rob Du Preez and Ewan Ashman.
But the Saints closed the gap after the break, with tries from Freeman and Matt Proctor as they cut the deficit to 27-19.
Sam Dugdale seemed to have made the game safe for the visitors but when Ashman was sin-binned, with Sale now down to 13 men, Saints scored a penalty try before further unanswered efforts from Callum Braley and Fraser Dingwall secured a remarkable win.
