"Our concentration in defence wasn't as good in the second half," Koeman acknowledged. "We created chances to score, especially at 1-0, but we lost our concentration.
"That's why we lost, and they closed us down well. We have to accept that we lost, but we're disappointed because this was a big opportunity.
"Now we have to be ready for Sunday. We've got five games left and everyone has a tough run-in. We have to win all five games to be champions."
The setback won't play on Koeman's mind for long, though. The Dutchman was keen to move his focus onto getting back on track.
"We've won a lot of games," Koeman added. "We've fought a lot. Despite today's defeat, we have to keep going."