Former Blues skipper Tom Naylor grabbed the only goal of the game as the National League high-fliers inflicted a 1-0 FA Cup embarrassment.
It was a thoroughly-deserved result for Paul Cook’s men, acknowledged by an honest Mousinho during his post-match address.
In a match also marred by first-half injuries to Regan Poole and Tino Anjorin, Pompey were dreadful, particularly in the second half as they chased an equaliser they never looked like getting.
And their head coach pulled no punches in his criticism of the players in their first-round elimination.
Mousinho told The News: ‘Firstly, I thought Chesterfield thoroughly deserved to win the game. They were the better side, even though we had the better of it and controlled the game up until half-time.
‘In the second half we were really poor, Chesterfield ran all over us. The final result is a really fair reflection.
‘Quite a few things disappointed me, without being able to nail everything down. I turned round to Jon (Harley) a couple of times (in the second half) and said “Sometimes it feels like we’re playing with 10 men here”. It’s the first time it has happened in a long, long time, so we’ve got to make sure we take it with a pinch of salt.
‘I was also disappointed with the quality we displayed in the final third. The lack of quality was probably the worst I have seen since I’ve been here.
‘We kept crossing the ball behind, we couldn't get shots off and, when we did have opportunities, we didn’t get at the target. I cannot think of one time when we tested the goalkeeper at all.
‘If you want to sum up the game pretty well, it was when we had a corner in the last minute and the goalkeeper came up – and we didn't deliver the ball into the box. That probably sums up the decision making, especially in the second half.
‘It’s easy to get carried away because of how poor we were in the second half, but the first half we were absolutely fine without really having too much cutting edge. We conceded from a set-piece, but the second half was just not good enough.’
Pompey now head to Leyton Orient in the Bristol Street Motors Trophy on Tuesday night (7pm).
Yet it is forthcoming League One fixtures against Charlton and Blackpool for which they will need to demonstrate a significant improvement.
Mousinho added: ‘We obviously can’t put those sort of performances in again.
‘If we do that against Charlton you will get punished as well. If you come back from the international break and do that against Blackpool you will get published.
‘Chesterfield are a good side, but these sides are probably slightly better and you have to make sure you are on it – and we just weren’t against them.’