Pompey fans could have been forgiven for thinking their side were waving the white flag when the teams were announced.
They made 10 changes for their Checkatrade Trophy trip to Southend, with three teenagers named in the starting 11.
Their hosts were much more experienced, but fell behind to a Louis Dennis header after just 93 seconds, while Gareth Evans completed a comfortable win on the stroke of half-time.
And Joe Gallen was impressed with the way in which the youthful Blues booked their place in the quarter-finals.
Pompey’s assistant manager said: “We made a lot of changes from our usual starting line-up and were up against a very strong Southend side.
“But I thought we played very well – two good goals, a clean sheet and on to the next round. It was extremely pleasing.
"Southend didn’t have too many chances and that’s always the starting point for us."
“Dan Smith would say he’s a centre-forward, but he slotted in at right-back and put in a decent cross for the opener.
“Louis headed it in well and that’s his first goal for us, so we’re happy for him and for the combination between those two players.
“Gareth’s a very fit guy and is having a great season for us. He had a slightly different role than usual and it was a clean finish – he strikes the ball brilliantly with left and right foot.
“We had certain rules to abide by with picking the team and also wanted a bit of experience with so many youngsters.
“Southend didn’t have too many chances and that’s always the starting point for us, so well done to everyone for the defending, which begins from the front with our pressing and works back.
“We expected to be up against it more in the second half, but it was fairly comfortable and it was pretty good going for us.”
The draw for the last eight – which is not regionalised – takes place on Sky Sports’ EFL Matters show from 7pm on Thursday.
Gallen said: “Anyone at home would be good, but that hasn’t happened for us too often in cup games this season!”