Chris Boyd says a shoulder injury was the reason Piers Francis was replaced at half-time of Saints' win against Lyon on Sunday afternoon.
Francis, who was making his first appearance of the season for his club following his return from World Cup duty, enjoyed an impressive first 40 minutes.
The England centre helped his team to secure a 19-0 lead at the interval, and they eventually won 25-14 in the Champions Cup opener.
But Francis could play no part in the second half, having been replaced with Matt Proctor, who was making his Saints debut.
And after the game, Saints boss Boyd said: "He (Francis) just took a bit of a ding to the shoulder so there was some medical intervention.
"I'm not exactly sure what the discussion was about his injury when he came off so I can't comment on that."
Francis was one of five World Cup stars who were making their first appearance of the season for Saints.
Courtney Lawes, Lewis Ludlam, Cobus Reinach and man of the match Dan Biggar also impressed from the start.
And Boyd said: "I think they all did (play well) and that's because they're all good men.
"Guys with a good character will come back and know that it's really important for Northampton Saints that they as senior players and leaders put in a pretty good shift.
"Right from Courtney and Luds at the front, Cobus Reinach, Piers - they all did a job for us.
"I'm really pleased for them."