Kenny Jackett assessed his 39-day stay at Rotherham and admitted: It was the wrong move at the wrong time.
Jackett is readying to face his former club on Sunday having surprisingly left the Millers shortly after his arrival last November.
That left the club looking for their sixth manager in 14 months, with the outfit 11 points adrift at the bottom of the Championship.
Rotherham finished a whopping 28 points from safety as they fell out of the division.
Now Jackett is preparing to face the club live on Sky Sports on Sunday (2.30pm).
The Pompey boss accepted the blame for the move not working out.
Jackett said: ‘It was the wrong move at the wrong time. I take responsibility for that.
‘You have to move on and move on quickly. I have to concentrate on the job I have here and am pleased to have.
‘Did the parameters move? No not necessarily. I take full responsibility.
‘It didn’t work out. I read the people wrong and I read the situation wrong.
‘So unfortunately I had to move on quickly.’
Jackett insisted he holds no malice towards Rotherham or their owner Tony Stewart ahead of meeting up with them again.
He has also been impressed with current boss Paul Warne, who is assisted by ex-Pompey manager Richie Barker.
Jackett said: ‘I wish them well. There’s no bad feeling on my part. They’ve got Paul Warne now who’s someone I knew very briefly.
‘He’s a terrific man. Both he and Richie Barker will do very well there for the club.
‘They are trying to establish themselves in this division after relegation and get back to the Championship.’