Alan Dickens says the decision to withdraw George North from Saints' team to face Sale Sharks was a 'precautionary measure'.
North warmed up with his team-mates and walked off the field with them to the changing room, but he was replaced in the starting line-up by Tom Collins, with Juan Pablo Estelles taking a place on the bench.
North had not started a game for Saints since October due to a knee injury, which he aggravated on his comeback against Harlequins at Twickenham on December 30.
He was able to play a part off the bench in the defeat at Saracens last weekend and Wales were keen for him to get more game time before the start of the Six Nations early next month.
But Dickens, who confirmed North's late withdrawal was nothing to do with his previous knee problem, said: "George North pulled out just before kick-off.
"He was just tight so it was a precautionary measure to hold him back.
"He's probably had a tough week training with Wales.
"It was unfortunate, a last-minute withdrawal so not ideal."
Stephen Myler was forced off at the break against Sale, with James Grayson coming on and kicking nine points in the 24-20 Saints success.
"Stephen had a tight hamstring," Dickens said.
"It's something we need to look at on Monday.
"It was fantastic for James Grayson coming onto the pitch. He kicked well and took his points.
"It was disappointing for Stephen."