Ian Sampson was unhappy with the second-half performance of his Northampton side as they went down 3-1 at Shrewsbury.
A brace from the impressive Jake Robinson secured victory for the Shrews after Kevin Thornton's free-kick had cancelled out Matt Harrold's 12th-minute strike.
Sampson said: "Our confidence was up at half-time after getting the equaliser but while we could argue about the second goal - we thought it was offside - our minds were on other things and we were still arguing with the referee and so got done with a third.
"We had lots of possession in the second-half but didn't really hurt them, picking the wrong pass at the wrong time, but hopefully they're a young side and they'll learn from their mistakes.
The Cobblers boss added: "Shrewsbury will certainly be in the top three at the end of the season and rightly so with the money they have to spend and the players they can attract."