City were 3-1 up and in control of the game, before three goals in the final six minutes of the match saw Southampton head back to the South Coast with an unlikely three points.
It was a chaotic, error-strewn final period, something Harsley says his side cannot afford to repeat, and he promised to continue working - both individually and collectively - on the squad’s ability to manage a game.
“I’m absolutely devastated, no other words for it,” he said.
“We can’t quantify what happened there, really, to be so comfortable in a game and collectively contrive to make so many errors and crumble.
“It’s something that happened on a couple of occasions last season and we thought we had done a lot of work on in pre-season trying to make it better. Obviously, we will continue to do so.
“It’s rocked us and we thought we’d moved on from this. We’ll have to address it.
“We spoke to them afterwards obviously but we want them to go away and look at it themselves and moving forward we want them to know they can’t do that in a first-team environment.
“Collectively we have to get better.”
It was a disappointing opening match of this season’s Premier League 2 campaign, but Harsley believes the broader picture remains much brighter.
A number of EDS players were involved on the first-team’s summer tour of Asia, with the likes of Tommy Doyle and Taylor Harwood-Bellis impressing.
Indeed, Pep Guardiola once again singled out Harwood-Bellis for praise on Friday, describing his performances on the tour as “so good” and suggesting he is ready for more involvement at first-team level.
Harsley hopes to see his players continue to be involved in training sessions with the first team as this season continues.
“The bigger picture says it’s all going in the right direction,” he said. “We’ve had lads on the first-team tour and were playing.
“Lads are getting better all the time and training with the first team. Hopefully we get more appearances this season.
“We will keep working with players and getting them better – long may that continue.”
“Taylor’s done well. He’s only really, really young.
“He likes to compete. He’s a bit different to other lads we have, and he can play as well.
“We want to keep pushing him and he, along with quite a few others, should have a chance.”