London - Southampton will be without Morgan Schneiderlin for the trip to Champions League spot rivals Arsenal on Wednesday.
Despite defeat at the hands of Manchester City, Southampton remain third on the English Premiership table ahead of their trip to the Emirates.
Koeman criticised his side for the individual errors he believes were responsible for the loss to a City side reduced to 10 men for the last 16 minutes.
The Saints' hopes of returning to winning ways have not been helped by the loss of midfielder Morgan Schneiderlin due to injury.
"He's not ready for Wednesday," said Koeman.
"He's now having a scan on his right leg, we have to wait and see the results of the scan.
"We know exactly what Morgan means for our team, he's one of our key players and without key players it's always difficult for the rest of the team."
Southampton have already tasted victory at the Emirates Stadium this season when they knocked Arsenal out of the League Cup.
"The previous result gives the confidence that we can beat them, but Arsenal in the last two games have had two good results," said Koeman.
"I think they are a little bit back with the spirit about these two results.
"It will be difficult, but you always get space against Arsenal. They like to play, they like to play offensive.
"I think it will be an open game and, with our possibilities, we believe we will have chances for a good result," added the former Dutch international.
Sixth-placed Arsenal host third-placed Southampton at the Emirates Stadium on Wednesday at 21:45.