The south-coast club conceded two goals in four first-half minutes and Hasenhuettl said they could not risk a repeat of that performance against Watford as they sit five points above the drop zone with four games left.
"I think we have the quality to win there (at Watford) or to take a point, but we need a really clinical and perfect performance," Hasenhuettl told reporters.
"The message must be that a little bit of playing football is not enough for us, so we have to be aggressive again.
"We have to work harder than we did on Saturday in the first half because otherwise against Watford, a very strong team, it is again a big problem to take something."
Hasenhuettl will be without injured defender Jannik Vestergaard for the trip to Watford and he will make a late call on Yan Valery who is recovering from illness.
"It will be difficult I think for Jannik," Hasenhuettl said. "He had big pains in the training session on Friday and stopped in the training session.
"With Yan he had a high temperature in the night so we drove him home and we will have a look. There is not a lot of time between the games so we will have a look."
Hasenhuettl could change Southampton's formation, especially in defence where he has alternated between a back three and a back four in recent games.
"As always I think we know that we are flexible when changing shapes," he added. "We showed at Newcastle again that we can immediately change the shape and get more comfortable."
(Reporting by Rohith Nair in Bengaluru, editing by Ed Osmond)
