Saints secured two wins in their pre-season double header at Franklin’s Gardens on Saturday - but the day was marred by a serious injury to Tom Stephenson.
The 22-year-old sustained a broken leg during the second half of the second match of the day, against Rotherham Titans.
Stephenson received lengthy treatment before being replaced, and he had an operation on Sunday morning.
It is a big blow for the club and the player, who missed the second half of last season due to a foot injury.
The centre was expected to put real pressure on the likes of Luther Burrell and George Pisi for a starting place during the forthcoming campaign.
But Stephenson will now be forced to endure another lengthy spell on the sidelines.
Flanker Ben Nutley also picked up an injury as he was forced off just 10 minutes into the day’s first friendly, against Cornish Pirates.
Saints beat the Pirates and the Titans convincingly, with several players making their first appearance at the Gardens.
Louis Picamoles was one of them, with the France star, who joined from Toulouse during the summer, starting against the Titans.
Saints won the game against the Championship side 52-5 thanks to a dominant second-half display.
First-half scores came from Luther Burrell and Stephenson, with Lee Dickson (2), Lewis Ludlam, JJ Hanrahan (2) and George Furbank all registering after the break.
Dickson was the man on kicking duties, with the scrum-half impressing from the tee.
Saints had earlier beaten the Pirates 35-19, with tries coming from Alex Waller, Stephen Myler, Dylan Hartley, Harry Mallinder and Howard Packman.
Saints team v Cornish Pirates: Tuala; K Pisi, G Pisi, Mallinder, Packman; Myler, Kessell; A Waller, Hartley (capt), Hill, Paterson, J Onojaife, Gibson, Nutley, Harrison
Replacements: E Waller, Fish, Beesley, Denman, Parkins, Peters, Cheishvili, Marshall, Bennett, Groom, Olver, Green, Dingwall, Strachan, Furbank
Saints team v Rotherham Titans: Foden: Estelles, Stephenson, Burrell, Collins; Hanrahan, Dickson; Ma’afu, Clare, Brookes, Moon, Craig, Wood (capt), Ludlam, Picamoles
Replacements: Beesley, Fish, Denman, Parkins, Peters, Cheishvili, Marshall, Bennett, Groom, Olver, Green, Dingwall, Strachan, Furbank