Saints fell behind 30 seconds into the second half to the first of two Rodrigo goals, but a series of substitutions laid the foundations for a dramatic late comeback.
Joe Aribo pulled a goal back following a cross from fellow substitute Adam Armstrong, before another replacement and another home debutant, Sékou Mara, provided an exquisite assist for Kyle Walker-Peters to level the scores in the 81st minute.
“I'm sweating – not only because of the heat, but because of the intensity of the game today," Hasenhüttl said, as temperatures hit 30°C at St Mary's.
“In the end, I think it is a deserved point because we showed we can react. It's not always easy when you're two down to turn things around but I think the subs did a very good job today, and we were immediately on them.
“In the second drinks break we had to adjust a little bit, and after these changes we were in the game.
“We were speaking last season about how five subs will be a game changer, and especially when they have temperatures like today it's even more important to bring some fresh players.
“This is why they are so important for us. I said in the moment that everybody has to do his job, and also the guys who worked until that moment have the responsibility for winning a point today. It feels good, but we have to keep on going.
“We are trusting in ourselves and the way we are fighting, and everyone did a good job today.
“It's not always possible to come back after conceding two goals, so we have to be very critical because this is not possible all the time.
“It was important, especially after the first game, to come back here and taking a point can be a good first step in the season."