Ben Thompson has urged Pompey not to dwell on their first defeat of the League One campaign.
Gillingham won 2-0 at Fratton Park on Saturday as the Blues’ disappointing home run continued.
Tom Eaves and Alex Lacey netted in the first half to inflict a maiden loss in 12 matches for Kenny Jackett’s side.
Thompson impressed having appeared as a half-time replacement for Ben Close, but the Gills deserved their win.
And he wants the table-toppers to swiftly bounce back – with a trip to AFC Wimbledon next in the league programme.
He said: ‘It wasn’t the worst performance, we have played worse than that and won games.
‘We have to keep training hard and keep working on our game, there’s no reason we can’t stay at the top of the league.
‘The ball was going wide, over the bar, hitting players, they were throwing bodies in front of the ball, defending for their lives, but it wasn’t meant to be. Hopefully next Saturday we will put that right and get the three points.
‘In the heat of the moment in the game and you come into the changing room and not is going to be happy, you are never happy when you lose, if you are happy at losing you shouldn't be here.
‘Everyone is down and disappointed, but it has been a fantastic start to the season, we are top of the league after 12 games, the best position we could possibly be in.
‘There is no way we can drop our heads after one defeat in 12 games. It’s not something you can dwell on, we've got to go again.’