David Ripley says the Steelbacks were 'naive' and 'too defensively minded' during their eight-wicket defeat to Lancashire Lightning on Sunday afternoon.
Ripley's side were well beaten, losing by eight wickets at Emirates Old Trafford.
Richard Levi made 40 for Northants, but no other batsman got going as they were limited to a meagre 123 for nine from their 20 overs.
And Lancashire made light work of the reply, getting home inside 16.1 overs to claim a comfortable Vitality Blast victory.
Northants have now lost all three of their games in the competition and sit bottom of the North Group standings.
"Richard Levi fell to a blinding catch but from then on I thought we were a bit naive," said head coach Ripley.
"We should have plotted our way to 150 or 160 and that would have been competitive.
"Poor batting set us up to lose that game fairly comfortably.
"We had two lads caught off long hops and that’s poor execution but we were also too defensively minded.
"We needed to hit the balls into gaps and run better than we did - we needed a more positive intent.
"We weren’t smart at the beginning, we weren’t smart in the middle and we missed a big finish at the end.
"We bowled and fielded pretty well, which is good because we haven’t done that so well over the last couple of games."