A late fightback by the Championship side ensured that the Sixfields friendly ended in a 3-3 draw but Mackay knows his side will struggle in the league this season if they repeat their first-half showing.
Quoted in the Watford Observer he said: “I thought first half we were very slack and sloppy and Northampton came out and did what I expected Northampton to come out and do and that is go out there and, with heart and determination and passion and football ability, go and give us a tough game and we didn’t respond in the first half.
“We had some words at half-time and we showed a bit of passion and desire in the second half which is something that we’re going to have to do a week on Friday night [at Norwich City], so they’ve got the message.”
Nuneaton host the Cobblers on Tuesday before Saturday's game against Birmingham City.