The current Republic of Ireland manager was in charge at Villa Park for four years before departing the club in the summer of 2010, achieving three successive top six Premier League finishes.
Since then, Villa have gradually declined and this season ended with relegation to the Championship.
Commenting on their current situation, Mortimer felt the downfall at Villa Park started with O’Neill’s exit in 2010 as he felt the former Leicester City boss should have stayed on.
“If O’Neill was a great coach then why would he want to leave a project he had started?” Mortimer was quoted as saying by the Mirror.
“That began the downfall at Villa. From then on it was all about change your manager, lack of team cohesion, buying players who didn’t fit the system, teams that didn’t function properly, no manager getting to grips with what the club is all about.
“Why O’Neill couldn’t have given it half a year, half a season, to see whether it was going to work I don’t know. Or even fulfilled his contract. There was no pressure on him.”
O’Neill’s exit at Villa was largely understandable considering the club had sold James Milner and the resulting funds weren’t put back in to the team, but what followed was simply quite bizarre.
The Irishman ended up at Sunderland 18 months later, a team who were at the time showing very few signs of ambition in terms of breaking in to the Premier League’s top six.
Villa have suffered from a succession of bad choices since O’Neill’s exit, and it may be some time before they bounce back to the top-flight as a major rebuild is needed at Villa Park.