Kenny Jackett knew what was required following a goalless first half at Fratton Park.
Southend’s diamond midfield meant that more width would be needed in order to break them down.
The visitors should have scored just before the break after being awarded a controversial penalty, only for Nile Ranger to blaze it over the bar.
So Jackett brought on Matty Kennedy and, within nine minutes of the restart, he provided the cross for Brett Pitman’s winner.
The Blues boss said: “I’m delighted with the victory. It was hard-fought and we had to dig deep to overcome a good Southend side.
“A key moment in the first half was the save from Luke McGee and then there was the penalty decision.
"I felt that we played a lot better in the second half and really came into our own. The substitution to bring on Kennedy really proved the difference."
“The linesman gave it for a handball by Matt Clarke and I couldn’t see from where I was whether that was right or not, but it was missed in any case.
“I felt that we played a lot better in the second half and really came into our own. The substitution to bring on Kennedy really proved the difference.
“He was an outlet for us against a side that packed their midfield, so getting the ball out wide was going to be important for us.
“We couldn’t find Jamal Lowe as much as we wanted to do and it had to be on the other side with Kennedy.
“He offered us a very good attacking threat and then provided the key moment with the cross for Brett’s winning goal.”
Pompey have bounced back from a four-game losing run to win each of their past three fixtures in all competitions.
Jackett said: “The players have dug deep and I’m really pleased with that. We’ve also got more to choose from now, which has provided some competition.
“Some experience has come into the team, as well as a bit of freshness. It can be fine margins between a win and a loss, but it certainly feels better after the first one.”