Keith Curle revealed Cobblers midfielder Shaun McWilliams was taken off as a precaution after he experienced 'palpitations' during Saturday's 2-0 win over Newport County.
The 21-year-old made his ninth successive start against Newport and was enjoying another strong performance before he went to ground on the halfway line eight minutes into the second-half.
Following several minutes being tended to by the club doctor, it was decided McWilliams couldn't continue and he was replaced by debutant Egli Kaja.
And Curle revealed exactly what was wrong with his midfielder after full-time.
"He was suffering from chest pains," said the Town boss. "He saw the doctor at half-time and I said it's probably because it's the hardest he's worked all season!
"But he was taken off as a precaution because he's been having palpitations. The doctor thought he was going to be OK but we've had a situation here recently (Leon Barnett) and we've got to be cautious.
"The player's concern and the player's welfare is paramount. The medical department said if it feels any different to when you've settled down at half-time then you've got to tell us and you've got to come off. We're mindful of the player's safety."