The club sits 17th in the English Premier League, three points above the relegation zone, and Hasenhuettl said there were issues with the squad during the off-season.
"We will be active (in January), that's for sure," Hasenhuettl told reporters ahead of Thursday's league trip to Chelsea. "We had issues in the summer - the balance of the team is not 100% what it should be.
"We have a few positions where we have a lot of alternatives and other positions, we don't have any. This is not healthy when we have suspensions or injuries.
"The problem is the (transfer) fee for Premier League clubs is always very high," said Hasenhuettl, adding: "We will try to bring two guys in."
Hasenhuettl confirmed the south coast club would be without striker Shane Long and winger Moussa Djenepo for Thursday's match in London but midfielder Sofiane Boufal could return to the starting lineup.
SOUTHAMPTON, ENGLAND - NOVEMBER 30: Danny Ings of Southampton reacts to referee decision during the Premier League match between Southampton FC and Watford FC at St Mary's Stadium on November 30, 2019 in Southampton, United Kingdom. (Photo by Naomi Baker/
Image credit: Getty Images
"I have one big issue with Shane Long because he slipped last weekend and has a little knee injury," the Austrian manager said. "He couldn't train today and it doesn't look very good.
"Moussa, it's still too early for him to play against Chelsea. Kevin (Danso) was ill but was training today for the first time with the team. The good thing is Pierre (Hojbjerg) got hit on his knee but has recovered very well so could train.
"Yan (Valery) is still out, but Sofiane (Boufal) is better and could train without an injection so is an option."
Southampton face a Chelsea side that returned to winning ways over the weekend with a 2-0 win at Tottenham Hotspur.
"We had a very difficult game at home against Chelsea (a 4-1 loss) and we know how strong they are," Hasenhuettl said.
"They struggled a little bit before the Tottenham game but it would be interesting to see what shape they will use. They played with a back five and it worked well against Tottenham. We must be prepared for both shapes."