The right-back turned out for the Blues in the 1-1 draw at Pride Park earlier this month in the knowledge his mother had terminal cancer.
She subsequently passed away 48 hours later, with the former Arsenal youngster having been granted leave of absence on compassionate grounds.
As a consequence, Swanson was unavailable for the trip to Barnsley and last weekend’s 2-1 win over Lincoln.
However, he returned to training on Tuesday to put himself in the frame for Saturday’s visit to Wigan.
And Mousinho has praised the 22-year-old for his Pride Park ‘bravery’.
Pompey’s head coach told The News: ‘We spoke about Zak’s family issue last week and, now that the time has passed and he is back in, it’s probably an appropriate time to let everyone know that unfortunately his mother passed away.
‘It was on Monday before the Barnsley game, so thoughts with Zak and his family. The squad has rallied round him brilliantly well.
‘While it’s not something which needs to be public knowledge, I think it’s appropriate to make clear Zak played on the Saturday against Derby with the full knowledge that it was probably a matter of days before his mother was going to pass.
‘I thought it was incredibly brave from Zak, he put the team first – and then we managed to put family first after that.
‘That was an outstanding performance under the circumstances. Zak’s only a young lad, he is 22, so to do that is credit to him.
‘It’s a very sad time and gives us a bit of perspective in terms of what we are dealing with.
‘A few people were probably wondering where Zak was and, as I said at the time, he would have been in the squad for the Barnsley game.
‘We felt it was much more important for him to go and deal with this – and he will be a much stronger character coming out of the back of it.’
Prior to his mum’s passing, Swanson had started four of Pompey’s last five matches, impressing in Joe Rafferty’s absence.
Subsequently, Rafferty has featured at right-back in the League One leaders’ last two matches.
He added: ‘The most important thing was he spent some time with his mum during her last days.
‘As a head coach, that’s something you never imagine you’re going to have to speak about – and it hits you really hard.
‘All these lads we’ve got under our stewardship, they are just young lads, they are footballers, we have to get the best out of them and try to make the most of their careers, but they are people first and foremost.
‘Families are ultimately the most important thing and I am sure everybody in football recognises that.’