Absolutely this.
The system so far has been to recruit players that have either just or in the last few years been released by a Premier League academy. This often means they are decent on the ball but can lack the grit required for league 2 (and the NL - but got away with it due to the quality of the opposition).
A bit of a re-think might be required or we may need to splash some cash on a couple of defenders in the summer.