Kenwyne Jones dedicated Saints' final day win to stricken skipper Claus Lundekvam.
The Norwegian centre half was carried off just before half-time with an ankle injury.
He now looks almost certain to have to sit out the club's play-off games.
Jones revealed that Lundekvam's injury spurred his teammates on to go out in the second half and grab a win against the doomed Shrimpers.
He said: "Claus is our captain and having seen him carried off after that we couldn't let him go off and not win the game and get in the play-offs.
"We had him in our hearts and came out in the second half and did it for him."
Jones scored twice to help his side to a 4-1 win over Southend at St Mary's - to take his seasonal tally to 16.
He was struggling to contain his smile at a play-off place.
"Before the game we knew we had to win and score goals," said the Trinidadian striker.
"There was definitely a little bit of nerves once the game started and Southend got the early goal on.
"But once we levelled it, we were on top and took the game to them after that.
"The way we won gives us a lot of confidence.
"The support was so great, so come Saturday if it's like that again (against Derby) it's going to be a massive boost for us."
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