The 19-year-old was forced off after just 20 minutes of the Gallagher Premiership game, which Saints went on to lose 36-17.
And he now looks set to require surgery, which could keep him out for the rest of the season.
"Fraser's got a high ankle sprain and he's probably going to require surgical intervention which will keep him out for a reasonable chunk of time," said Saints boss Chris Boyd.
"He'll probably be out for six or eight weeks and the likelihood now is that his goal will be to get that right so he can go away and play for England Under-20s at the Junior World Cup."
But there is better news on fly-half James Grayson, who was forced to withdraw during the warm-up at Allianz Park last weekend.
"He didn't train today but we're pretty certain he'll be right for the weekend," Boyd said at Tuesday's media session.
"He took a blow on the hip and was a bit tender but there's every indication he'll be available for the weekend."
Prop Paul Hill was replaced midway through the first half last weekend due to concussion.
But ahead of Saturday's Premiership clash with Bristol Bears at Franklin's Gardens, Boyd said: "Hilly's fine.
"He failed his concussion based on his actions on the field but he hasn't had any symptoms at all.
"He looks to be fine."
And more good news comes in the form of Piers Francis, who was forced off with concussion against Bath on February 23.
"Piers is available for selection," Boyd said.
"He's cleared out all his injury concerns and he'll certainly be available."
Saints have been without the influential Teimana Harrison since he was replaced inside the first 10 minutes of the Premiership Rugby Cup semi-final against Newcastle Falcons on February 9.
And Boyd said: "T is 50/50 at this stage.
"That hamstring has been a little bit more problematic than we'd hoped.
"But we've been lucky that our loose forwards have been going pretty well and if T returns it will be a great bonus. If not, we've got plenty of soldiers in that space anyway."
Several players picked up injuries on the artificial surface at Saracens last weekend, with scrum-half Alex Mitchell posting a picture of his badly grazed leg on social media.
So what does Boyd think of pitches like the one at Allianz Park?
"They are what they are - they're a sign of the times and a modern approach," he said.
"We're pretty pleased that at Franklin's Gardens we play on grass and that would be our preference.
"It would be fair to say a couple of guys have got some pretty nasty burns from the weekend, but life goes on."