Jim Mallinder says Tom Collins was withdrawn due to cramp against Saracens on Saturday.
But Saints are still waiting on the severity of Mitch Eadie's leg injury.
Eadie, who joined from Bristol this summer, was forced off just 15 minutes into his competitive debut after a lengthy stoppage.
Collins was taken off during the second half of the 55-24 Aviva Premiership opening-day defeat at Twickenham.
And Mallinder said: "Tommy Collins has just got a bit of cramp so he's fine.
"I was really pleased with him.
"You do look for positives and he was very lively and made some lovely breaks.
"Mitch Eadie had a fantastic pre-season and it was disappointing to lose him early on.
"He's just got a leg injury and we're going to have to see how that is."