Saints have suffered an injury blow with the news that Ken Pisi requires shoulder surgery and could be out for up to 12 weeks.
The Samoan wing was forced off just 20 minutes into last Friday's 25-18 victory against Harlequins at Franklin's Gardens.
Pisi dislocated his shoulder and will undergo an operation on Monday to repair the damage.
He has been replaced in the side by Taqele Naiyaravoro, who makes his Saints debut in the No.14 shirt against Saracens at Franklin's Gardens on Saturday.
"I thought Kenny was just coming into his game nicely, but he's had a shoulder dislocation and goes in for an operation on Monday," said Saints boss Chris Boyd.
"We would expect him to be out for around about 12 weeks.
"But T (Naiyaravoro) has been brought here to do a job for us, he's itching to play and he's going to get a run-out tomorrow.
"He's a power-based player, a big man who runs hard, and we just need to find ways to get him into the game.
"Generally with big men you try to get them into the tight contest but he's a great space runner as well so if we can mix it up, that would be great."
Another player who has been ruled out this weekend is hooker James Fish, who picked up an ankle injury in the game against Quins.
Dylan Hartley takes Fish's place in the team, with Mike Haywood on the bench.
"Fish has done a great job for us in the past couple of weeks but he sustained an ankle injury early in the game last week and he's not quite right," Boyd said.
"We're blessed with four good hookers and a young hooker in behind so there's no point in pushing Fish at this stage.
"Dylan has chipped away in a couple of games and earned his chance, and Mikey Haywood, who has played a lot of games for the club, needs to get back to his football."
Boyd has opted to select Ehren Painter at tighthead, with 20-year-old prop taking the place of Ben Franks.
It will be Painter's first start in Saints' first team after making five appearances from the bench.
"It's great news for him," Boyd said.
"He's a big slab of a man, he's raw, he's got a lot to learn and this is a start for him against one of the best looseheads in the world in (Mako) Vunipola.
"He sees this as a great chance to test where he's at."