It was a moment of controversy that might have cost Pompey a point in their League One match at Peterborough.
Reeco Hackett came off the bench to halve the visitors’ deficit, only for referee Robert Madden to disallow the goal following a discussion with his assistant.
Hopes of a comeback were sparked late on when Nathan Thompson found his own net, but there was little time to strike again.
John Mousinho explained the reasoning he was given for the decision, although remains unconvinced that it was the right outcome.
The Blues boss said: “I got an explanation from the fourth official and I understand the fact they don’t have the benefit of replays.
“He’s given it for offside against Dane Scarlett and having watched it back, it looks like it was a very tight call.
“You’re probably not going to be able to say whether it definitely was or wasn’t, although the fact that the assistant hasn’t put his flag up tells me everything that I need to know.
“My message to the boys when I thought we’d scored was that there was plenty of time left in the game.
“I just felt like the momentum was in our favour and we did get the goal eventually to ensure it was an interesting finish, but unfortunately it was too little, too late.”
"You’re probably not going to be able to say whether it definitely was or wasn’t, although the fact that the assistant hasn’t put his flag up tells me everything that I need to know."
Mousinho conceded that his side had been second best for the opening hour of the London Road contest.
He said: “It’s obviously disappointing because of the way the game ended – we showed a lot of character to come back into it.
“We have to take a lot of positives from that, but if you look at the first 60 minutes, Peterborough were the better side and deserved their lead.
“We didn’t get the press quite right and they had a lot of joy in the first half, so we made some tweaks and did it a lot better.
“There were a couple of times when we got caught on the counter in the second half, though, because we were perhaps a bit too open.
“It’s not always going to be plain sailing, but Peterborough are a good side and I thought they deserved the victory on the balance of play.”
Louis Thompson made his first start since suffering a broken leg in August, getting through 58 minutes before being replaced by Hackett.
Mousinho said: “The fact he’s been out for so long and hasn’t had more than half-an-hour, I thought he put in a really solid shift.
“Against one of the better sides in the league, he gave us absolutely everything and we can build on that.”