Michael Appleton refused to blame Johnny Ertl after his Pompey debut despair.
And the Blues boss revealed Josh Thompson should have been playing instead – only for paperwork to scupper his plans.
Ertl was pressed into central defensive duties against Oldham on Saturday, deputising for the suspended Kevin Long.
However, Thompson’s one-month deal was registered too late on Friday for him to be considered.
The former Celtic man’s absence infuriated Appleton, who declined to go into detail when pressed on the reason for that hold-up.
As it was, Ertl diverted Jonathan Grounds’ left-wing cross past his own keeper in the 59th minute for the only goal of the game.
The former Sheffield United utility man was eventually substituted in the 83rd minute, capping a miserable Blues bow.
But Appleton believes the 29-year-old will bounce back to prove his Pompey worth.
He said: ‘It’s an own goal and a lapse of concentration more than anything.
‘To be fair, Johnny got in the right position but he just couldn’t sort his feet out – the poor lad.
‘It’s one of those things that can happen if you put the ball into the right area – you can force defenders to make mistakes.
‘Bringing him off later was no reflection on scoring the goal, we just felt we had to get an extra forward on to try to get something out of the game.
‘Johnny probably wouldn’t have played – and that’s the most annoying thing.
‘That makes me frustrated more than anything.
‘Josh Thompson would have been playing but that is something I don’t really want to go into at the minute.
‘I think the less I say about it the better.
‘It was down to paperwork – international clearance was not the problem, you can get that up until the last minute.
‘Don’t get me wrong, Johnny’s comfortable in that position.
‘But I would have liked to have eased him into it a little bit more rather than having him in from the start.
‘You could tell he is lacking a little bit of match fitness and sharpness but over the coming weeks and month he will certainly get that.’
Overall, it was a tremendously disappointing result for Appleton’s men.
Oldham had endured a poor start to the campaign, yet managed to inflict Pompey’s first home defeat of the campaign.
After an impressive opening, the hosts failed to carve out enough clear-cut chances.
As it was, Cristian Montano struck a post in the second half for the Latics.
The defeat has left Pompey winless in five matches this season.
They play Bournemouth in the Johnstone’s Paint Trophy tomorrow night.
Appleton, though, was not too critical of his side, despite the Fratton Park scoreline.
He added: ‘It’s difficult for me to really criticise the players about the performance.
‘Oldham defended really well, they defended deep and made life difficult for us to break them down in and around the box.
‘If anything, we could have been a little bit more precise in the final third and created more clear opportunities than we did in the opening 10 to 15 minutes.
‘But I can’t really fault their effort and enthusiasm, they kept trying to get back into the game. Even when it was 10 versus 10 we were the ones trying to get the goal and get into the game but Oldham were happy with taking the 1-0.
‘I am not really going to criticise my players.
‘I can’t really criticise them, other than at times we were giving cheap free-kicks away and allowing Oldham to put balls into our box.’