Southampton were unbeaten against the Londoners last season, drawing both their matches. This term Jose Mourinho's Chelsea are down in 14th place having already conceded 14 goals in seven games.
"Maybe they have some defensive problems," Koeman told the club website on Thursday.
"Last season they were the team, along with our defenders, who had the best record in defence. They didn't concede a lot of goals.
"This season it is different. Maybe the confidence is not quite the same."
Southampton are 10th in the table after last week's 3-1 victory over Swansea City and Koeman wants more.
"We have to play better...than we did against Swansea, be even more clinical," he said. "But there is confidence in the team, the physical state, the mental state, of the team is high."
Chelsea have lost five games this season in all competitions, including Tuesday's 2-1 Champions League reverse at Porto.
"We know they are a difficult opponent," added Koeman. "For sure they will react after the loss against Porto...and they will go up the table.
"We know the qualities they can bring and we have to be on a very high level to be able to have a good result."
(Reporting by Neville Dalton; Editing by Tony Jimenez)
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