The Blues were forced to play the final 71 minutes against Leyton Orient with 10 men following Sean Raggett’s red card.
Regardless, goals from Christian Saydee and Abu Kamara secured a 2-1 win at Brisbane Road to earn Pompey a place in the knock-out stages of the Bristol Street Motors Trophy.
It was an impressive reaction following Sunday’s FA Cup defeat at Paul Cook’s side, representing one of the upsets of the first round.
And with 11 changes at Orient, Mousinho was delighted with the response in testing circumstances.
The head coach told The News: ‘One of the things we felt coming into the game is we needed to respond.
‘We had come off a first loss in a long time on Sunday, losing a couple of key players to injuries, and then, in this game, losing Sean to a red card.
‘It felt like a lot was going against us in a game we didn't necessarily need to win to qualify, but we wanted to win and put in a good performance.
‘It was tough with 10 men, especially as the first shape we tried didn’t work and then we changed that really quickly. As soon as we changed that, things started to work a lot better.
‘It says a huge amount about this squad, I am really, really proud of some of the performances out there, there are so many factors you can look at behind everything.
‘Denver Hume hasn’t started a game since April and played 90 minutes. He didn’t look like he was struggling, which says everything about his character and what he has done over the past few months to keep himself fit.
‘You had Kusini Yengi on his second start off the back of a long injury, coming back and physically looking brilliant. At the back end of the game he was still working the channels.
‘Harry Jewitt-White has hardly played a game on loan to Havant & Waterlooville, yet he came in and had to play a couple of positions, while also having a good involvement in the first game.
‘Then it was just the way the lads made sure they kept the ball out of the net in the second half, barring the goal.
‘We had to dig in at times and defend the box, but we did it – and the character has shone through.’
Raggett was sent off in the 19th minute for bringing down Theo Archibald, although the red card is competition specific, meaning he is available for Saturday’s visit of Charlton.
And Mousinho had no issue with Stephen Parkinson’s decision to send off his captain.
He added: ‘I thought it was a sending off and I was frustrated with how it came about.
‘We had a throw-in down our side and decided to abandon the middle of the pitch which is where the header dropped down to, then they won the second ball and broke from there.
‘It was probably a poor decision trying to go tight with the centre-forward, who then spun off Ryley’s shoulder and Sean unfortunately cut across him from behind.
‘I didn’t actually have any complaints with that red card, to be honest.’