Despite progressing into the play-off round of the Europa League, Saints drew at Newcastle on the opening day of the Premier League campaign before losing 3-0 at home to Everton on Saturday.
Southampton welcome FC Midtjylland to St Mary's on Thursday night looking to take one step towards the Europa League group stages - but manager Koeman is still striving for defensive cohesion after a second summer of upheaval.
The Dutchman is currently without injured goalkeeper Fraser Forster and full-back Ryan Bertrand while Morgan Schneiderlin and Nathaniel Clyne have been sold and Toby Alderweireld opted for a move to Spurs over making his loan move to Southampton permanent.
That has left the south coast club with a lack of defensive stability - something Koeman highlighted as a cause for concern.
"Expectation was different last season than for this season," he said.
"I knew already maybe there was a problem because, if you talk about the defensive strengths of the Southampton team we had last season and a big part of keeping cleans, you are taking about certain positions - goalkeeper, four defenders and two controlling midfield players.
"Some asked me about Schneiderlin - he was just one of five different players (missing) in those positions last Saturday
It's normal that you will lose something - the new players need time in that position, the team needs time to develop and have the same strengths as last season.
"If you're missing five of the seven, that is one of the reasons we have already got five goals against us in two Premier League games
In Europe we showed our strengths in offence against Vitesse (in the last round) but tomorrow's game will be more difficult.
"You need really good organisation, we lost some qualities to that organisation but I believe in the new players, we need time and we need to develop and learn, we need training sessions.
Koeman batted away any questions about potential moves for Celtic's Virgil van Dijk or Kalidou Koulibaly of Napoli - but he did admit he was still chasing another addition.
"We need one player
A defender," he said
Having seen Premier League rivals West Ham fall at the last round having fielded a weakened team, Koeman remains committed to taking Southampton into the group stages and will not compromise on team selection ahead of Sunday's trip to Watford.
"I see this as one of two games against Midtjylland to reach European football," he said of Thursday's opening leg.
"That the main reason I don't pick the team with Sunday's game in mind - I pick the strongest team in the best system to make sure we have a good result tomorrow."
Source : PA