Manager Kenny Jackett is delighted with the first-team impact of the central defender following three-consecutive starts in the competition.
Casey has formed an encouraging partnership with Christian Burgess, on Tuesday night the pair keeping a second-successive clean sheet for the Blues.
The towering 19-year-old, who has spent loan spells with Gosport and Weymouth this term, was handed his Pompey debut in last month’s Checkatrade Trophy clash with Arsenal under-21s.
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And Jackett has been impressed with the youngster’s ongoing development.
He said: ‘Burgess was very, very good against Peterborough – and young Casey as well.
‘This has been a good competition for Matt, he has grown and progressed as the season has gone on.
‘No disrespect to Gosport, I think he is better than that standard of football and has a lot of potential and ability.
‘He’s wiry at the moment, but if you can picture him in his very early 20s then I think he’s going to be a good player.
‘Matt has good composure on the ball as well and, although is quite quiet at the moment, has good self-confidence.
‘I’ve seen him develop through the season, these games have been good for him. He’s faced two sides that have put out very strong teams in Southend and Peterborough and acquitted himself well.
‘He is one of the positives in the competition so far.’
Casey’s most recent loan spell at Gosport was ended at the turn of the year.
With Checkatrade Trophy and FA Cup fixtures ahead, it was decided to bring the Academy product back in-house.
Although Jackett is unsure whether Casey will be given more loan time away from Fratton Park in the immediate future.
Pompey’s boss added: ‘We’ll see. We are not flushed with players necessarily and at present are still in three competitions, so we’ll have to keep moving and progressing.
‘We always want to try to win, to compete and promotion is our aim, that's our priority, but also the development of young players is important at any club.
‘We want to still keep developing some younger players, some home-grown players, and for the likes of Matt, Bradley Lethbridge, Haji Mnoga and Dan Smith, they've had valuable time in this competition.
‘That will only put them in good stead in the future.’