Paul Paton admitted he was ‘surprised’ not to have been offered a Pompey contract.
The Northern Ireland international trained with the Blues earlier this month, after being released by St Johnstone.
But Kenny Jackett opted not to table the midfielder a deal at Fratton Park.
Paton subsequently signed for League One rivals Plymouth Argyle, making his debut off the bench in their 3-2 win over Bristol Rovers on Saturday.
The 30-year-old thought he was going to be offered a contract at Pompey before moving to Home Park.
Speaking to the Plymouth Herald, Paton said: ‘I was at Portsmouth and never signed.
‘I was a wee bit surprised.
‘I had a week’s training there so that has stood me in good stead to come here.
‘Apart from that, I have just been doing a lot of track running.
‘It has not been nice – it’s hard to motivate yourself – but I’m glad I did that because I’m feeling good now.’
Paton could come up against Pompey when they travel to Plymouth on April 14.