Given the upcoming Christmas football schedule, Hasenhuttl has quickly put his plans into place to try and get his team playing his brand of aggressive, fast-paced football.
And Romeu, who seemingly fell out of favour with previous manager Mark Hughes before his departure, says he and his teammates are already reaping the benefits.
“It’s been tough,” Romeu said per the Daily Echo. “On Wednesday we had a recovery session and everyone was feeling hamstrings, and our quads were all tight, but that’s part of the change.
“We’re going to get that. There are different exercises that are hard for us to get used to it but after a week or two it will come naturally.
“It will affect us for a few days and weeks but when we get that physical fitness right the team is going to play with the higher intensity he wants for 90 minutes.
“It’s about getting the ball as soon as we recover and getting that pressure into players all round the pitch when in tight areas.
“That’s difficult, therefore you need to get ready for it. So everything is short, there are lots of sprints, and to get there as soon as possible we need to do it every day.”
Romeu says the atmosphere of the Southampton squad has changed after just a week as Hasenhuttl instills some discipline.
“I knew as soon as he came I had my chance and getting physically stronger helps me,” Romeu continued. “I feel more comfortable, we all feel better and welcome that change into our game.
“Off the pitch we’ve made a few rules, he’s brought everyone together and things we didn’t pay attention to before we’re paying attention to now.
“Everyone’s on the same page and a lot more positive and optimistic and that’s very important in a group when you’ve got so many individuals. Habits are getting better.
“You can feel how the atmosphere has changed in one week.”