Chris Boyd gave an injury update after Saints' 36-17 Gallagher Premiership defeat at Saracens on Saturday afternoon.
Saints were forced into a reshuffle before the match at Allianz Park because of fly-half James Grayson's withdrawal in the warm-up.
George Furbank filled in at 10 with Ahsee Tuala switching from wing to full-back and Tom Collins coming into the starting 15.
And when asked about Grayson's hip issue after the game, Boyd said: "It was a bit of a blow.
"He took a bit of a bump a couple of days ago and we thought he was going to be fine but in the warm-up he just felt uncomfortable.
"We had to move George (Furbank) to 10, which is not a natural position for him, but I thought he did a really good job in that space.
"We had to shuffle things around a little bit but I wouldn't use that as an excuse."
Fraser Dingwall appeared to be forced off with an ankle problem during the first half, while Paul Hill came off with concussion.
"Fraser trained really well with us but he took a blow during the first half," Boyd said.
"We don't expect him to play for us for a few weeks now."
Saints decided not to call on Dan Biggar for the trip to Saracens due to injury.
"Dan has had a bit of a shin and knee problem," Boyd said.
"He's been a mixture of being with Wales and with us, but the injury is still problematic for him so we decided he wouldn't play."
Dylan Hartley has not played for Saints since December 21 due to a knee injury and he had surgery in a bit to rectify that on Tuesday.
"Dylan had his operation on Tuesday and he was very bullish after it," Boyd said.
"The surgery was relatively minor but he doesn't want to put any pressure on himself in terms of having a time-frame for a return, and neither do we.
"They opened his knee up and found a couple of bits that shouldn't have been there but overall it seems very positive."