Jim Mallinder confirmed that Courtney Lawes suffered an ankle injury in Saints’ 20-15 win at Saracens on Saturday.
Lawes was forced off five minutes before half-time and was unable to put any weight on his left leg.
James Craig came on in his place as Saints proceeded to earn a crucial 20-15 Aviva Premiership victory at Allianz Park.
But there will remain concern around Lawes, who had been released by England earlier in the week.
“He’s certainly sore,” said Saints boss Mallinder.
“He’s twisted his ankle and we don’t quite know the extent of the damage as yet.
“He got a bang to the arm as well, but I think that’s okay.
“The ankle pain is more than his arm.
“He’s a tough lad is Courtney. He doesn’t come off if he can help it and we’ll have to wait for the next 24, 48 hours.”
Tries from Lee Dickson and Stephen Myler helped earn the win for Saints, who have now won four of their past five games.
And Mallinder said: “Our performance, particularly in the first half, was very good.
“We opted to play into the wind, played well and we were disappointed to go in behind.
“But with the wind behind us after half-time we expected to get ahead, and we did that.
“We scored through Lee Dickson and were disappointed we couldn’t quite score again immediately with Ben Foden because that would really have put us points up and made Saracens to have to play more against us.
“As it was, they came back and they’ve got a cracking kicking game, particularly from nine, putting us under pressure, where we didn’t help ourselves.
“But we’re generally happy to come away with the win.”