The young striker was part of the first team squad that arrived back in the UK from Valencia in the early hours of Friday morning but asked to play at Roots Hall later that evening.
Andros Townsend and Tom Carroll also swapped Spain for Southend and all three played their part in what was an impressive display against the League Two side.
Harry led the way with a hat-trick with Souleymane Coulibaly, Andros and Jon Obika also on target.
“I had a long day but you want to play these games,” he said. “You want to play 90 minutes, keep match fit and to score goals as well is great.
“The main thing is fitness, sharpness and at the same time, trying to impress the manager.
“The team was incredible. To be honest, Southend couldn’t get near us, the way we played, the way we moved.
“We were clinical in and around the box and fortunately I managed to get a hat-trick.
“As a striker you always want to score goals and as a team we looked so comfortable, it was great to be involved.”