Southampton won five of their nine Premier League games -- including a 1-0 victory against Manchester City -- after the resumption of the 2019-20 season following the COVID-19 hiatus.
But Hasenhuettl's side have failed to pick up where they left off, falling 1-0 to Crystal Palace in their Premier League opener before Wednesday's loss to Brentford.
"At the moment some players are not at the level we have seen them in the past," Hasenhuettl told reporters. "We had no pre-season, no time to work together and we are physically at a different level than when we came back after lockdown.
"Definitely we are not in the position at the moment to play our game. It's very intense, we need to have fit players. There's definitely a lot of work to do."
Southampton host Tottenham Hotspur in the Premier League on Sunday. (Reporting by Arvind Sriram in Bengaluru)