"Danny had made a big step after 20 minutes to the ball and then sprinted and he then had a problem with his hamstring," Hasenhuettl told reporters ahead of the league clash against Cardiff City on Saturday.
"We need to have him here, but at the moment, we have to know it will take two-three weeks.
"But the good thing to know is that Pierre-Emile Hojbjerg is back after his concussion, which is important for us because you saw against Burnley we missed him and his experience in this position and it's good he's back."
Despite Ings' absence, Hasenhuettl believes Southampton are not short of goalscoring options - citing midfielders James Ward Prowse and Nathan Redmond's purple patch in front of goal which have helped the club climb out of the relegation zone.
However, the Austrian is concerned about their leaky defence, having kept just one clean sheet since he took charge in December.
"I don't think we are showing at the moment that we have a problem to score," Hasenhuettl said.
"Redmond is in a very good moment and scores nearly every game. We have other players who can score like Prowsey for example as well.
"We should be focused in the future to have more clean sheets. Sometimes it has to be enough to score once, not two or three times to take a win."
Southampton, currently 16th in the league table, host Cardiff, knowing a win would see them move five points clear of the drop zone.
(Reporting by Hardik Vyas in Bengaluru; Editing by Christian Radnedge)
